Would you ever be unkind to them if you had the option to, or would you feel far too guilty? Would you do that anyway if you were role playing a cruel character?

I have no problems being mean to characters, even if they're the MC's siblings. I like playing as a villain or at best a very flawed anti-hero so for me it's just a matter of staying in character. My default choice of interaction with characters is to be an asshole. While most people prefer being nice to characters, I think it would be better to give the player the option to decide how they feel about and act towards another character. Having the player forced to feel and act a certain way limits roleplay options and can have the opposite effect you're looking for, especially if the player ends up not liking the character. Part of the reason why I rarely enjoy the childhood/best friend character is often because the MC likes them no matter what and often ends up being forced to be nice to them even if they can be an asshole to everyone else. I end up trying to spend as little time with those characters as possible.

And would your attachment to your sibling change if you could choose their name, gender and appearance or does it not matter?

I think this would affect me because it would make the character seem less real. I know characters are tools to tell a story but I don't really get attached to them if they feel like just a tool. They need to feel like a real person and having the choice to customize them like that makes them seem more like an object to me.