Can someone help me?

How old is she?

If shes young, its most likely she just doesn't really understand. I think she just might to mature a bit until she will.

I agree ^ it might just be based on how old she is または her maturity level. If she's only 8, she's probably being a little kid, kids say stuff like that. And you're siblings. Lots of sibs try to argue with each other, especially the little one. It might also have something to do with what she's heard around. Maybe someone's told her stuff like that's 'bad'? It might be what she's been taught によって other people. If your parents support LGBT, they might be able to help. In any way, try to teach her not to be so prejudice, または ask someone like a school counselour what to do. No matter what, don't take her opinion too seriously. If you're bi, that's fine. Be proud of it!
Hope it works out! :)