Wait? Didn't she stop liking MC because she considered him pathetic and not cool??????????? But it's okay for the other guy????? To be honest all of the childhood "friends" girls are for the streets
She mostly did stop liking him, hell she is even pushing back on him. Its hard to get rid of all those residual emotions, but lil sis has seen him in a new light and is now aware of what he is actually like as a person. She has also become more aware of what her actual stepbro is like and has realized he is the one that cares for her genuinely.
At this point she basically just wants closure and to be done with it. She wants to confess, get rejected, cry about it and have her actual family spoil her for a bit to then move on.
Notice how the last couple chapters she has been standing back just observing. Also how she called the MC oniichan on the bus ride and he called out how she never does that at school.
Right now she is just bluffing and is scared to go confess knowing the result but has to do it for her own emotional well being. Actual big bro knows this and is giving her a push todo so, supporting her all the way and also knowing whats going to happen.