Are you retarded?
Rachel isn't even the MC's mother - and not in a "let's dummy out this incest so that Patreon won't ban us"-way, either.
The MC's actual mom is dead, and that's always been a major plot point. Rachel was nothing but the MC's dad's live-in girlfriend after that. Hell, Rachel never even got married to the MC's dad, so she technically isn't even a stepmom. And because they were never married, the house technically belongs to the MC rather than Rachel.
All of those things play a big role in the story, and they've been part of the plot from the very beginning. Rachel is not the MC's mom, and she was never supposed to be either.