I said I'd ramble about the TMNT:HME sibling dynamics so here it is:
Some of this stuff I've already touched on before, but I'll try and go into a bit more detail on the specific relationships
Leo is the oldest, but all the brothers know he's not the favorite. If anything he's the least favorite ( Not the problem child, though, we'll get to that ) - though Splinter swears he doesn't pick favorites.
As far as all the brothers' views on him as a whole it's something like ' he's the oldest but sometimes he acts way too much like Splinter' and they've said this to him occasionally. And every single time it hits like a fucking brick.
Leo wonders if Mikey thinks he is cool or not - he doubts it. They're sort of in a ' in this together' mentality when it comes to Donnie and Splinter. Leo tries very hard to teach Mikey all about gender/sexuality without Splinter's knowledge. He wants Mikey to know that there is a safe almost-adult for him to come to about that stuff. Mikey doesn't really get it, though. Leo is a bit sad that they don't have a more healthy sibling dynamic - he could never, ever discuss big feelings with Mikey or talk about Splinter. It kinda sucks, but he knows they don't have to talk about big feelings. They just have to have fun.
Leo, for a long time, had a special sort of animosity towards Donnie because of him being the favorite. When they were much smaller, Leo actually hated him for it for a while. But as he got older and they drifted apart, he realized that it's not him vs Donnie, its all of them vs Splinter. He still thinks Donnie is a kiss-up, though, and he constantly tries to help him grow out of that.
Leo and Raph were a lot closer when they were kids, but the shift to disliking Donnie due to favoritism also kinda roped Raph into it, since Raph and Donnie are a lot closer than any of the other brothers. Nowadays, Leo is mostly just....sad about his and Raph's dynamic. He feels guilty for a lot of the way Raph grew up, he knows a lot of it is his own fault ( and some specific things that are his fault 100% that there will be a comic about later ). Overall their relationship is tainted by guilt - Leo will never forgive himself for treating Raph the way he did when they were younger.
Donnie is the favorite, as all the brothers will agree ( Raph still isn't sure that's true ) and Donnie won't deny it. He's proud of it, actually. He knows he's the favorite because he's the most well-behaved. He does his chores, he shows interest in Splinter's hobbies or what he's saying. Overall, he cares a lot about Splinter's opinion - definitely more so than his brothers.
Donnie has noted that Leo acts like Splinter, but he frames it as somewhat of a good thing. He's very.... calculated with his and Leo's relationship dynamic. He tends to keep his distance emotionally, and will call out when Leo is being an asshole. He has also been the most troublesome when it comes to Leo telling him to do chores. He is Team Splinter all the way.
Raph, as his twin, is his favorite brother hands down. Donnie does play favorites and Raph is his favorite brother. They spend the most time together, Donnie prefers hanging out with Raph than hanging out with the rest of his brothers, but they still have fights sometimes. Mostly over room stuff or Raph being too loud. No fight has ever lasted more than a day, though.
Mikey and Donnie have a weird, standoffish sort of relationship. They don't have a ton in common, but still like to play video games together! Mostly mine craft. Donnie just doesn't feel like hanging out with him too often is all :/
Raph has a complicated relationship with Leo- on one hand, Leo can be really cool and fun!! On the other, he can be as annoying as Splinter. And Raph easily gets mad when Leo makes fun of him, sometimes blowing up at him or his brothers for it. He's very easy to anger and sometimes Leo pushes his buttons on purpose. But to be fair, Raph pushes Leo's buttons back just as often.
Raph and Donnie are closest, but Raph sometimes wishes he was the older twin so maybe he could be the favorite. However, Raph usually reaps the rewards as well, since Splinter doesn't even really know he exists most of the time. If Donnie gets a treat he will find a way to get Raph one as well.
Raph and Mikey have an unspoken rivalry - Raph and Mikey have a lot in common, but Raph hates that Mikey is 'better' at so many things than him. He has his own hobbies, but Mikey always does them better. And he's afraid of his little brother surpassing him.
Mikey looks up to Leo as his ' cool but sometimes lame older brother' - most of the time he thinks Leo is cool but sometimes Leo will say/do something so lame that Mikey has to literally walk away. Mikey has also told Leo that he's acting like Splinter the most.
Mikey sort of looks up to Donnie, but it's mostly for his smarts. Other than that, he also resents Donnie a bit for being the favorite :/ Thinks he cares way too much about Splinter's opinion
Mikey doesn't know how Raph feels about his talents. They're close the way youngest siblings are close - they used to roughhouse a lot as kids and nowadays they compete in stupid competitions around the lair :)
So that's how the boys' see each other! I talked about this a bit before, but this is a bit more in detail since someone suggested it the other day <33
