The MCs whore GF values her sister's opinion quite highly. I expect this variable would influence how receptive she would be to an open relationship herself.
Her interactions have been tweaked considerably over the course of development, so I don't think it applies any longer.
There are less choices involving her than there used to be.
right - and to tell you the whole origin plan:
We originally had 3 different routes in mind:
1.) Stay loyal and refuse to be in OR - this path was abandonned in very early state as people didn't like her character and how she behaved.
2.) End relationship - which is implemented
3.) (actually my favorite one *sniff*

) GF will turn into a cuckqueen (many phone and video calls hinted towards that). But people still hated her and too few people managed to look behind the scenes and got the hints. So we had to decide to cut that path out and focussed on other girl's paths to avoid unnecessary toxic discussions and to please our supporters, as they gonna keep the business going on.
PS:
Despite whatever others thought could happen, it was never ever even slightly thought of her going netorare behind MC's back. Even she had a hall pass, she would have never ever made use of it - be assured.