Can you tell us more about Madara’s relationship with his others sibling? If I recall well, he has two brothers (other than Izuna) and one sister? How does he feel about their death?
This, for me, is a request for some angst before hashimada week, and I shall deliver.
Madara's siblings, it's true I don't talk about them often despite having a lot of headcanons about them. I subscribe to the popular theory that Madara and Izuna are named after two of five mountains, so I took thoses to name the rest of the siblings. It makes them three boys and two girls, but I did kill off the last one with Taiyō.
To me, Madara is the eldest, so, after him comes the twins
"Fake" twins, they don't look that much alike, but were strongly attached to each other. Togaku was more soft spoken and meek, where Myoko was bold and outgoing. Togaku passed at six on his very first mission and Myoko gained her first tomoe from this and her personality hardened significantly.
Kurohime was the last little sister Madara should have had, and Izuna's first younger sibling. She was however deeply burnt and born premature, so she died shortly after her mother, despite the effort put into keeping her alive.
Madara never fails to mention her as well when speaking of his siblings, even naming a crow summon after her.
Taiyō died during labour, she remained undefeated on th battlefield, but her health was already declining before. Madara was the only one able to withstand the heat his mother's body had surrounded itself with, as a last attempt at survival.
Now, Madara's relationship to his siblings.
I'll start with Kurohime, since she's the easiest. Madara grieved the relationship he could have had, and he remembers her. Her death didn't quite affect him as much as his mother's, since he had a very close bond with Taiyō and didn't get to know Kurohime at all. He still prays for her when visiting his family grave.
When it comes to Togaku, he was the first one to go, some elders tried to calm Taiyō down arguing that, at least, the boy was spared from the worst of the world, and she almost burnt them for the simple remark. The boy was very naive, especially because of how Madara kept speaking of peace, relaying a similar message as their mother. He believed there was good in everyone and was very nervous about his first mission.
Madara was the one who found the wounded unit, but, because of his inability to use medical chakra, none of the members were saved. They had been ambushed and barely escaped, sadly not surviving the encounter.
Sometimes, Madara thinks that, if it had been anyone but him, that day, maybe Togaku would still be alive.
Because of this, he was also afraid Myoko would resent him for not being able to save their brother, her twin baby brother. She didn't resent Madara, especially since he didn't act like nothing happened, unlike most of the clan who, after too little time of grief (in her opinion) moved on.
She became scared for Madara, however, seeing as he started hesitating in battle to land the last blow.
Her words were harsh, but she just cared.
Such a shame her words would come to haunt him in the end.
She loved all of her siblings, but had a soft spot for Togaku especially. She looked up to Madara, wishing to reach his level one day. This means she was also very competitive.
Fun fact: one day, in winter, Madara was playing with snow by himself and Myoko decided to throw a snowball at him. Surprised and a bit annoyed, Madara decided to counter attack, forgetting his mother's instructions to not throw snowballs, and before she could stop him, a ball of snow covered in a hard shell of ice was thrown right in Myoko's face, breaking her nose.
Madara felt so bad about it...
It's because of his high body temp, compensating for the cold, but also just melting the ice he touches, meaning that, when he thew the snowball at Myoko, its surface had melted and hardened back with the fast cold air, making it harder than it should have been.
Alright, that's all of the Uchiha siblings story for today, I hope it answered your question.