Sounds like sister has a personality disorder (sociopath, look it up on the internet and see if behaviour matches what a sociopath does).
She, or they would be gaslighting you to get a rise. Just ignore her when she does that (hurts her the most as is not getting attention). The smirk on face is common with sociopaths when they think they have it over you, or know you are on your own and not going to get help from anyone.
It comes down to how they were raised, where parents are too busy to spend time to bring them up properly. They don't crack down on the behaviour. They do that behaviour to get attention from parents, but flows onto other people as they get older.
They project a lot too, so what they say about you, is about their problems, not yours. If think that way you'll understand why they are doing it.
Just have to learn about what they are and to adapt.
should know more than me on subject of Bipolar and might have some advice of his own.