NAH I'm presuming she knows you have a bf? Randomly kissing your friend is not the best way to tell them about your sexuality. She dropped a lot shocking information on you in a very shocking way.

From my understanding you were also not in any way protesting her sexuality but rather the fact that she kissed you out of nowhere, disrespecting your relationship in the process.

I also think she could have come out to you and told you about her feelings in a way that wouldn't have put you in such an awkward position, the you could have let her down easy. But I don't want to call her an asshole either cause she's just shooting her shot and probably feels super rejected. If she was the one who told everyone she'd be the a but since you think that's on the sister. She might only have part of the story and therefore judging accordingly.