Why are you positive shes going to over indulge while she has S4? Has there ever been an issue before where she has endangered S4 because of her drinking?

If not I wouldn't just assume she is going to put her child in danger. It comes across as questioning her parenting ability and if it is the case I would be asking why you haven't looked into this already.

Shes an adult and until she gives you a legitimate reason to think shes going to put S4 in danger I wouldn't jump to conclusions.

Accept what is, let go of what was, and have faith in what will be