Never tried harem route, but since sister is forced upon you idk how the dev gonna handle endings.
Hopefully you can dump her eventually otherwise you basically trapped in relationship with her.
The convo you have when you look for her after she run's away kinda hint at the fact you can finally stop this thing but later MC still kisses her.
I still remember when the game had a gameover if you rejected her when she jumps you at night, dev removed that choice now so you can't reject her and that fine but i hope we get more definitive choice in the future to be just siblings.
I personally don't care for her in that way and she kinda ruins any other relationship by just existing, she is nice but i prefer Rachel.