I honestly don’t see the issue with them getting into a relationship. For starters, they’re not blood related so there’s nothing wrong biologically. And secondly, it’s not like they grew up together, so there’s really nothing wrong there either. If they grew up together and MC naturally sees Ayase as his sister then yeah I’d say it’s weird. But in this it feels like he’s forcing himself to see her that way because he automatically thinks it’s wrong to like her.
Now going back to my “growing up together” point, the longer MC waits to confess these feelings and get with her the more it’s gonna creep into weird territory. Go ahead and accept the feelings and confess them now while the two of them are still in this “we just met and are more friends than siblings” phase.