See, I disagree with you about Mia.
She was never a bimbo. Every single time(Except the Dr. visit...WTF) she figured out all the MC's tricks by herself. So she likes sex a lot. A very important point. Like almost every brother and sister relationship in the world it's a love hate relationship (Or any siblings for that matter).
She is the opposite of stupid IMO. That is why I like her. I also don't think she minds that much if she get's tricked, since she love the sex. It's more the idea that she was tricked then the actual sex that made her mad.
So his initial exploits, she was angry about her brother tricking her into sex, but also not angry since she enjoys sex and she still loves her brother (As a brother, nothing romantic at the time). Over time she started enjoying their sex more and more and as things tend to go, her feelings for him start to grow until such time that she fell for him completely.
In truth, the MC also fell for her...he just had to go duck his mom and fuck it all up!!