Older Sister Problems HELP!!

I'm currently dating someone with a rather complicated family situation. Their father left them while they were young， and it seems like my boyfriends older sister never stepped out of that shadow of their father leaving them.

She's very closed off to people and very negative. Ive been dating my bf for 2 years now and things don't seem to be changing with the sisters attitude. My bfs mom likes me and is very understanding of how i feel ，bfs older brother and sister in law seems understanding of my situation with the sister too.

I'm just not sure if this is something I'm blowing out of proportion and it really shouldn't matter all that much whether the sister accepts me or not? Really，im just not used to anyone hating me to that extent without even getting to know me or talk to me. Ladies，what would you guys do if you were in my shoes?