@semiprecious All I can think to say is OMG F*** THAT GUY.
I hate people who feel entitled to comment on a stranger's life which they know nothing about. You sound like a great mom who has gone out of her way to help family out.
Confession time: I've always wished I came from a large family like yours with multiple siblings, cousins, and so on. I wish I'd had sisters and brothers to bond with, spend holidays with, and celebrate milestones such as weddings, graduations, and births with. I used to day dream about sharing a room with someone so I would never be alone, sitting at a table with tons of family at dinnertime, sharing secrets with cousins and siblings, etc. Unfortunately, it was just me and my one brother (we don't get along) and after generations of estrangements and fertility problems, I have no cousins to speak of. As far as I'm concerned, your kids and niece/nephew are living out my wildest dreams. You're a wonderful mom, and don't let anyone tell you otherwise.