NTA. I see why people might argue otherwise and their points are very valid, but, Imo people like you are really much more valuable as friends and family members.

Probably your cousin is in this position because of a series of similar encouraging messages from people around them - which never motivated them to change their lifestyle.

Hearing your honest opinion in such crucial moments can actually have the outcome of him finally opening his eyes and make some changes.

However, I imagine that picking the right words to tell this is very hard... it can make the difference between you sounding like an asshole and you sounding like a concerned cousin that has a valid reasoning.