Thank you everyone for the comments. (Please no more replies.)

I will try getting my family to seek therapy, but I am doubtful that my sisters will be willing. I will see to it that my younger sister is supported at least till she is 18, and my parents will decide depending on the situation then.

I see the point about how maybe there is more to it (psychological trauma etc). However the issue now is that anything we do/say that is in the slightest way unfavorable to them is seen as a hostile act. I am not sure how to reach out to them to try and understand the possible deeper issues they might have. My parents have tried all sorts of ways. I was always the 'nice older brother', only recently did I take a harsher stance (as my mum calls me crying about it).

We have tried limiting contact between my sisters as we see the bad influence. Did not work and only aggravated it; after all they are two fully abled (soon-to-be) adults. My younger sister goes over to my older sister's place so much that it is causing problems in her marriage.

This isn't a sons vs daughters gender issue; my younger brother is only 12, who knows what might happen. If anything, my younger sister was always seen as the brightest kid in the family. (and no, my mum didn't pressure for ridiculous straight As)

Indeed, it is impossible for me to give the full picture in one post – it spans years of conflict, and this is only my side of the story. However, I have questioned myself many times as unbiasedly as possible if my family really did do something so terribly wrong and I still come to the conclusion that my sisters are too much (even if my parents erred). On the other hand, I do not think that they have ever tried to reflect on their actions or our well intentions for them, or tried to reconcile the issues.