You sound insanely petty. This is none of your business, plus you say that this cousin (she's your cousin, right? your "aunt's daughter" is your cousin?) is otherwise respectful and kind. And I"m guessing you aren't eating dinner there every night, so you have no idea if this is a regular thing or might be related to some other relationship issue between them. And if you are over their every night, then I'd REALLY recommend backing off because this is not your drama.
Don't tell your aunt what to do, don't tell your cousin what to do, keep your thoughts to yourself, and constantly remind yourself "I don't really have enough context to be assessing this situation."