one of MC sister's is BI. You can't change that. Later, after maybe half of the game you have option to tell her that you are no ok with sharing and she stops. But before that she shows her lesbian tendencies and she is very interested in one of your potential LI. LIke you said - she mentions it and lusting after her
There is also her "pure lesbian" friend or lover(if you are ok with sharing). She appears later in the game, but if you have no sharing then she do nothing to your girls
Later, MC BI sister tell him story how she become interested in women, and you will see unavoidable lesbian scene with your and her sister. So you can guess she is also an lesbian or BI
In the end you have 2 BI sisters, both had fun with each other for a very long time in the past
yea, you can avoid anyone in this game
you need also to be careful. You can accidentally turning straight LI into BI, by picking wrong choice and letting her watch lesbian movie. Personally i find this situation ridiculous, this is really new level of idiocy