I don't think I would bring it up with the mom. You handled the situation well and by setting the SIL straight in front of mom, you already showed them both that nap time is non-negotiable. The mom never said she had a problem with the naptime, the SIL did.
Perhaps you could require an earlier drop off time so that this child is not just at your home for rest time, but some busy playtime as well before hand.