The LW talked to the grandmother (LW's mother) about the incident. Had the grandparents just used the wine glasses to drink the beer or otherwise drank less than the LW is assuming, the grandmother had an opportunity to say so. I'm just taking the LW's word for it that the grandparents drank too much, but of course I concede that the LW may be wrong in that judgment. I agree with you that two adults drinking five beers over the span of a few hours is not a problem.

The LW asks, "Do I need to get over this and let them babysit...?" (emphasis mine), so I don't agree that the LW has already decided not to let them babysit again. S/he is asking just that.