If you have close cousins, you can adopt those as niblings. We treat our cousin babies as if they're part of the crew anyhow (we've got like 12 including the two cousin babies, at that point it's like why the heck not include them?)
We have stupid family issues, so one of my aunts and her fam (my cousins, their wives, kids) don't really speak to us. My other cousin, whose little boy I love oh sososo much, lives in VA, so far away!
My friend has a foster daughter who I spoil the shit out of because I have no girls and no babies, and her bio mom is a POS.
Also, having grown nephews is alright, since they do fun stuff with us like go to shows, and also
(the grown ones at least)