C.C.
Well, this is a tough one. It's hard to tell your mother in law how to do things (when she is watching the child for free). If it is only when your daughter is sleeping and she is waking up dry, then it's clear your daughter understands about potty training and is not regressing. And your mother in law can let her take a full nap without worrying if the mattress is going to be soaked. (Recently my 5 year old had a night time accident and we had to get a new mattress... ugggggh...) If you are really worried about it, maybe you could get a rubber mattress pad for the bed that your daughter sleeps in while she is there, and offer to wash the sheets if she has an accident.