I'm not saying that there aren't parents that enable.
What I'm saying is that it's each parents business and no one elses. And if someone says that they don't care if they're mean because it's a pet peeve of theirs, explain to me why it's any of their business to be such a pet peeve? Sure, if it's in your family, or a close friend, but seriously, a pet peeve for the entire population?
And my point is that most times the 'best' medical advice you will get is "a child will eat if they are hungry enough." I was absolutely willing to test it. My "medical" professional would not give me parameters. What do I do if she still doesn't eat? How long do I let it go? Do I withhold formula or how much do I give her to ensure she didn't dehydrate yet would still be hungry enough to eat?
I KNEW my dd had a problem. I wasn't exactly thrilled at the prospect of starving my child to prove a point, but that's what the doctor wanted me to do. Without enough information to ensure that she was not harmed. When you have a child with feeding problems you're pretty much on your own. Through an incredible amount of patience (especially wrt know it alls), and practice and discussions with my dd, we worked it out with no help from the medical field.
Note: The was in the days of the infancy of the internet. By the time it was more readily available, my dd was 4, I'd found a new doctor that was willing to work with me, but she referred me to a specialist that was not. Becasue my dd was clearly not on the ASD spectrum, and she wasn't SID in any other way, he was unwilling to listen or help.
Sounds like your nephew's story is the exact opposite of mine. In my case a parent that knew something was wrong but it wasn't wrong enough to get help, in your nephes's case a child with something wrong, but the parents refused to help. I'm sure there are other aspects of your bro and sil's parenting that you don't agree with beyond enabling.