I've said it before and I will say it again (and happily chirp it until I am blue in the face) - Anyone is allowed to park in a P&C space. It's not illegal. Morally wrong, yes, but in reality they are not an entitlement like a disabled space is, just a guideline. You won't get a fine for parking in one and, frankly, its no one else's business regardless of whether or not they have kids.
Your cousin is very passive aggressive and, ultimately, wrong 