I think it all depends on the family. What would be unquestionably rude to a stranger can be perfectly polite within the family - depending on the family and the context. So, I wouldn't hesitate to ask my sister if she had used a gift I had given her, or why I wasn't invited to something, and I wouldn't hesitate to tell her she had brewed the coffee wrong and I couldn't drink it. But I wouldn't hesitate because I know that she would not be upset by these things, and I would not be upset by them either (for example if she had not used the gift). I would never do the same to a stranger.
I mean, why just leave coffee sitting around until it gets cold? It's going to get dumped, not drank, no matter what. The only reason to maintain the polite fiction is because to do otherwise would upset or hurt his mother, and I don't get any sense from the OP that was the case.
Etiquette needs to be flexible to be useful. So yes, I think the fact that it was family vs. a stranger makes a difference. I don't think that one can always label an action rude without understanding the dynamic, and this is definitely one of those cases.