I cannot fathom not having a "reliable" head count; with my family it could sway anywhere from 8 of us to 40 of us. That just blows my mind.
I don't bring anything when we go to my aunt and uncle's or my grandma's house for dinners. It just never crossed my mind to do so. My parents bring wine and beer for all of our immediate family and give my aunt and uncle money for dinner/dessert.
I don't know what age I might start bringing stuff, but my cousins do the same when they come to our side of the family's events. I've never even thought about it until you brought it up. It doesn't bother me. Perhaps it's a family dynamic.
If it bothers you, mention it when you invite them. "Dinner starts at 4, bring your favorite side dish, some wine or a dessert please..."
They are family, they can't possibly be offended that easily. Right?