My now ex-SIL was a nightmare.
She was late 30s and still lived at home, had a pigsty room, ate rustler burgers for breakfast and didn’t wash.
We’d only been together for roughly a year and my partners parents were on holiday and asked if he’d pop round and check on the place as she wouldn’t clean and leave plates everywhere. We went round after he’d finished at the gym and so he popped in for a quick shower, whilst I stayed downstairs. His sister came in and accused us (with her new boyfriend in tow) of having sex in the shower whilst she was out. She kept saying ‘with her’ and completely refusing to acknowledge me. It was the weirdest encounter I’ve experienced - she ended up telling her mother and acted as if we were actually shagging in the shower.
She used to feign weird illnesses, pretended she had bulimia for a while and would turn it on and off.
She had a late miscarriage which was so tragic and awful but she used this as an excuse to then turn everything to her so when my friend was pregnant, (which happened 4 years after her miscarriage) I was then throwing a baby shower for her and when asked about it by my FIL my SIL ran out the room crying!
FIL was just as bad he fed into her BS. MIL was better and once told me she felt trapped.
the partner was a dick too who cheated but of course that was my fault