I think Chika would have been willing to share Sensei with Yumi (and probably Niki)
The real issue is she thinks that
1. Chika believes he was cheating on her with Yumi, behind her back
2. Chika believes Sensei went to the beach just to spend time with Yumi
Honestly the second probably hurt her more than the first.
I mean yeah your boyfriend "cheating" on you with who is basically your sister, fucking sucks. But then you find out, while he is depressed and shit, as low as he can get, that he would rather spend time and the night with her than with you?
That will deal a severe blow at your self-steem.
I wouldn't be surprised if Chika starts blaming herself for not being "good enough"
Also now Chika, whether she forgives them or not, CANNOT find out about what Nodoka did. With the new information Chika has, it goes from {Nodoka tricked them and blackmailed them, causing Yumi to go on a rampage next day} to {Nodoka caught them red-handed and Yumi beat her to try silence her into submission}