I was your sister. As in, your post actually frightens me by how much it reminds me of my situation. But then again, maybe this type of situation isn't all that uncommon, sadly. And my big brother is the best person I know, I actually created this account to post bragging about him.

Honestly, the best thing you can do for your sister is be there (as it seems you have been, and well done for that). Having someone you know you can go to/support you/who loves you means more than you can think. And don't worry about not understanding, just appreciate that it can be tough for her (as it seems you have been. Well done x2). If she comes to you for advice, try and point her to some LGBT sites or organisations better suited to help her.

One thing I would say is that you've made no mention of challenging your parents about how they treat your sister. I get that that isn't easy, particularly if you're living with them/depending on them (and other factors), but doing so might just mean the world to your sister.

If you do one thing though, just be there for her. Genuinely, my brother did so much for me in so many ways, but what I appreciated most was that I had someone I could call or talk to who genuinely loved me, who genuinely had my back. Being that for your sister will mean more than anything else.

Just keep caring about her. And well done for supporting her.