I have a scenario in mind, let's say when Shu, Reiji, Ayato, Kanato, and Laito are still young children, Beatrix and Cordelia both died untimely deaths which leave the first 5 S bros motherless and no mother figure except for Christa, who doesn't go insane and while caring for Subaru, she suddenly has 5 other boys to take care of so she takes over. Counting her canon personality and the fact that she didn't go insane, I'd like some headcanons of the S bros raised by a sane! Christa Sakamaki.
===== CHRISTA RAISING THE SAKAMAKIS SCENARIO =====
===== CHRISTA RAISING THE SAKAMAKIS SCENARIO: SAKAMAKI =====
"As stressful as it has been, I am grateful I have been able to raise you all. I have learned a lot about each and every one of you. And I see you all as my own. Please never forget that. I have grown to love you all as my own sons."
=============== Shu Sakamaki ===============
"I won't ever forget you stepping up to the plate and being willing to raise all of us as your sons. Neither will I take advantage of it. I know the triplets in particular have always been hard to deal with. But I do genuinely appreciate you, mother."
Shu will probably be one of the first to start growing respect and a good relationship with Christa. While Beatrix favored Shu over Reiji, Shu didn't have a lot of respect for Beatrix unlike Reiji did. Mainly since he was a spoiled child who didn't really respect his authorities around him.
However, since Beatrix will have an untimely death, that will leave Shu probably in a realization that he should have treated his birth mother better, and feels like he might be getting a second chance by having Christa take him in as her own son. He would also have more respect for her taking in all of the older children rather than just solely raising Subaru.
Overall, his relationship with Christa genuinely might be closer than what he had with Beatrix. As a child, he might still retain some of his spoiled behaviors, but so long as Christa continues to help raise him in a more healthy light, Shu could very much become a more balanced and overall have a more healthy-mindset than what he would have with Beatrix.
=============== Reiji Sakamaki ===============
"Since Beatrix is my birth mother, I still feel some sort of loyalties to her. But I will acknowledge your kindness and acceptance of all of us as older siblings to Subaru, and raising us as your own. I respect you, and I wish to thank you. I never really did so within my youth."
Reiji, despite his hatred for his birth mother, also wants to stop at nothing to try and get her approval and attention. After an untimely death, Reiji might be shot mentally for a while. Mainly because now he doesn't know what his purpose could be, and also because he wouldn't respect Christa for taking him in as a child as much as Shu might.
Overtime, he will grow to appreciate Christa more for not giving up on him, and might genuinely not know how to feel with Christa showing him compassion, attention, and overall love that he never felt from Beatrix. His loyalties to trying to impress Beatrix probably would not go away for some time.
However, now that he is older and able to look at things much more logically, Reiji will have a lot of respect for Christa that he would wish to express moreso now than ever before. Especially since Christa raised him with much more attention he desperately craved from Beatrix, Reiji will have a lot of respect and wish to protect his Christa even more, and will eventually see her as his real mother now than he did as a child.
=============== Ayato Sakamaki ===============
"I know I'm a lot t' deal with sometimes... So I appreciate you not givin' up on me. You've never really made me do shit I didn't like doin' like that woman would. I don't really see her as being my mom. I think you are instead."
Ayato wouldn't know how to really process his mother dying untimely. There is a lot of hatred for Cordelia for never letting him have a normal childhood and all of the mistreatment he had to endure from her. However, having Christa suddenly scoop up all of the older Sakamaki boys as her own children will also kind of send Ayato for a loop.
Most likely, he'll be a nightmare to handle. Although Cordelia was also a nightmare within herself, he still would have some sort of desire to impress her and become the best just as she always told him. Although those perceptions and desires might lessen overtime as Christa continues to raise him, the damage from Cordelia was already done even as a young child.
While older, Ayato will have more respect and consideration for Christa. He might have genuinely tried to hurt her mentally and emotionally by lashing out and never claiming her to be his mother. But as he ages, so does his perceptions of her change overtime. He will see Christa more as a motherly figure, more healthy, probably than Cordelia ever would have been.
=============== Kanato Sakamaki ===============
"I don't really know why you decided to take us all in as your own children. At times I wondered if it was just to make Karlheinz happy or to fuel your ego. But I do realize that you have genuine care for all of us in your heart. So I suppose you're not completely worthless like my mother was."
Kanato as a younger child was in place just like his other two brothers just to please his mother. While her untimely death probably did not affect him as much as Ayato or Laito, he still probably not have the best reaction to Christa suddenly stepping up and saying she was now his mother.
He will overall be a very troublesome child, just like Ayato would be, and lash out a lot. Even from a young age, Kanato genuinely will probably try to beat and break down Christa mentally just because he doesn't want her to suddenly have claim over his life just like Cordelia had. Also in fear that he will simply just be used by her.
As he grows older, his tantrums probably will not disappear as they stemmed from the trauma of dealing with Cordelia as a young child. However, he will start to realize that Christa never gave up on him, even though she probably had every right to due to his temper and treatment of her. And he will most likely grow to care for her more overtime.
=============== Laito Sakamaki ===============
"I... don't really know what to say. I see you more as a mother to me than my own mother was. You've never made me do... unspeakable things like that woman did. You took us all in whenever you didn't have to, and I very much admire you for that. You've been patient with me, and understanding. I wasn't used to that."
Laito, despite the horrible relationship with his mother, probably will feel some sort of loss at her suddenly dying. As complicated as that may seem, trauma he endured makes him care about his mother. Even if it is only to serve and fuel Cordelia's ego, which is what she wants out of a relationship like that with Laito. However, although he is affected by her death, it is not to say he is devastated or going to lash out at Christa for taking him in.
He will feel weird and strange not having to have a relationship with Christa as he did with Cordelia. It might even come as a shock to him that once Christa finds out what went on, she would get upset, angry, and disgusted at that woman. Laito would need a lot of healing, and now that Cordelia is gone, he finally can. Especially since Christa takes him in with open arms.
Laito probably will grow one of the best relationships with Christa due to her care and worry over him and what Laito experienced with Cordelia. He might grow protective, and argue a lot with his triplet siblings for lashing out at Christa, whenever they do. And he might be one of the first to see Christa as his real mother.
=============== Subaru Sakamaki ===============
"You've always been so hard on yerself, mom. But don't ya' see you've raised six sons completely on yer own? Ya' didn't have t' take in my brothers, but ya' did. And I really appreciate ya' doin' that. They would've turned out even worse than what they were if they were left motherless."
Subaru would hold a lot of respect for Christa being willing to take in five more sons on top of himself to raise so suddenly. And although he might feel some sort of jealousy rise from time to time, he knows in his heart that it is a good thing she has taken in his siblings as her own sons as well.
He will probably be much less angry, now that Christa is in his life, not neglecting him, and helping raise five more sons on top of himself. They could have genuine days that feel like a big family, and Subaru would have desperately craved that should he not have been able to have it.
However, with this, Subaru is the most protective over Christa. Anytime his brothers like Reiji, Ayato, and Kanato might lash out at Christa or not respect her as their mother, Subaru will fight back twice as hard and make them regret it.