There is just something about brothers and sisters with a special, unbreakable bond that speaks to me. I have a few theories about that.
Someone who has known you their entire life, shared living space with you, knows all your flaws but still chooses you unconditionally, even being willing to break societal taboos for it.
A constant, steady presence that won't just vanish overnight, ghost or abandon or betray you.
Someone with similar upbringing and life experiences, who understands the burdens upon you, who will not judge you for not being perfect but will still encourage you to be the best possible version of yourself.
With my own family history being a mess, the notion of someone taking things to the opposite extreme and unconditionally loving and supporting and wanting you.
Clingy, sweet little sisters with a sassy side. Confident, nurturing big sisters who dote on you. There is something just magical about that.
And of course, the thrill of the forbidden. Though to me, such relationships always must be built on consent, mutual respect and mutual desire. None of that "seduce a sibling, do the deed, then fuck off and leave them alone with a dysfunctional family" bullshit. Looking at you, Aki Sora!