@descending_angel87: No, there was a very small but obvious hint that the mom know.
In ch 34, the mom changed from Kaori-chan (feminine suffix) to Kaori-san (gender neutral) one.
Mom only said for them to take a bath, not 'together'. i.e. she said it like the children should take a bath first, while likely knowing in her head that Nanami will keep a lookout given Kaoru's secret.
As for them sleeping in the same room, meh, clearly mom doesn't mind if she get a grandchild early, also given the personality of those two nothing is going to happen.