IMO you should talk to them. Your ultimate objective is to be present at your sister's funeral, right? You're not wrong for being angry at your parents, but I think it's also important to realise that they were the ones that brought you into this world and cared for you up to the point where you were "kicked out." As bad as they were, they're still your parents yknow? From what i understand, it's either they refused to believe that you didn't rape your ex or they didn't like the idea of you being bi, or both. Regardless, it's been 5 years and maybe time has softened their hearts a little. You don't have to talk to them after you attend your sister's funeral, but you should at least hear them out... even though they're kinda blackmailing you... Plus, they might really just want to have a heartfelt conversation with you if they won't tell you about your sister's funeral unless you talk to them.

You have every right to be angry at them, but family shouldn't be completely cut off imo. If you do end up talking to them, do expect disappointment as a precaution, because if you "expect disappointment, you'll never be disappointed."