Poor yo, he is in for a full mind break. Then Kei is going to peek up his broken pieces (al according to keikasu). On the other hand that sly Kei is using some heavy NTR logic with his whole (let's practice) speech hahahaha
Is not like there's any healthy option for Yo. It's clear Yui is getting close to him because she's emotionally messed up. Imagine if the guy you liked rejectes you and then starts cross-dressing to seduce some dude instead. That's what messed Yui up. Now she's trying to get close to Yo to cope with all that shit, but I don't think she loves him. Like, at all. It's more like she's looking for validation.
If the TL was going to have Yo's mom use "she" at the beginning she should've kept using it, it's so jarring to just shift the pronouns used for someone in the middle of a conversation