Ok when the father get's married to a woman that isn't the mother she becomes the son's step mother. For said woman to become his mother in law he (the son) would have needed to get married to the stepmother's daughter or son.
But on another note the mother mentioned 'she is a deep sleeper so she won't wake up easily?' that was so obvious a sign for him to do something it wasn't even funny.
Base +12, Maxer123 +6, Tristail +7, firedragon89 +14, JollyCoops +6
Aside from what others said about her being his stepmother, I don't think this deserves the rape tag. It was obvious she was expecting him to fuck her in her sleep.