I'm thinking of criticizing my young cousins' (13 and 15) weight (and how they were raised) in front of them and their mom.
Don't you think that's a bit disrespectful? Why would you do that in front of their mom?
I'm trying to find the causes of their obesity.
I get being concerned about them being obese, but you shouldn't really criticize them in front of other people. Their mom will be upset to hear you saying bad stuff about her children, and the kids will be embarrassed and upset. If you're really concerned say something in private to their parents.
That's what I try to do, I'm just going to be honest with their parents.
If your concerns are pure and with good intentions, then you can go ahead and tell their mom and see how you can help to make them eat more healthily. I hope you don't offend them while doing so and make sure that you recommend some eating habits that are not going to be expensive to implement.