What Mom's thinking: Hmm, in order to get a lock on Sakurako-chan as a daughter-in-law, those two have gotta start banging. Let's see what flashing some thigh does, hehehe.
Hermom purposefully using the same phrasing ("hanging out") right after she used it should have clued her in that her mom knew what was really going on. But it makes sense that she doesn't have enough social skills to recognize that.