I'm pretty open about either twin or little sister. If it's a little sister she would have always been protective of her and pretty tolerant of whatever she decides as long as she's not full good guy. To be fair, she'd be tolerant of a twin's choices too, but less overprotective.
Full baddie would make the two of them a terrifying force to be reckoned with, which would be fun. Being swayed would make for some interesting conflict with Theia knowing she has to hide some parts of her from the sibling she adores though, which is also fun.
Right now Theia is planning to elope, so she'd love a sibling she trusts not to tell their mother or disapprove.
Also, great PB thought, they look really good together.