What is the role of the step mom?

I'm the step mom. I love the boys, but they are night and day different. The 16 year old is tall, thin, athletic and popular. He loves sports and he is active. I'm a former college athlete so we bonded from day one. In fact, the running joke is he could be my biological child, not hers. The 10 year old is bright and artistic. He likes to read. He is also severely over weight (he just tipped the scales at 200 lbs!) and he is a couch potato. Not video games, just nothingness. He can sit and do nothing for hours. Walking the dogs is a real stretch. He usually makes it to the end of our driveway, then heads back. He is also a mama's boy. We get along well (I am also an artist and we are avid readers together), but I am secretly frustrated with his weight and immaturity. Mom gives him ANYTHING and EVERYTHING he wants on the weeks he is with her (50/50 custody arrangement). At our house there is discipline. His father is very strict with him, but he is a guy. The little things go under his nose unnoticed. Last night we went for dinner with another couple. Kids were with us. The 10 year old spent the entire evening chewing with his mouth open and interrupting the adult conversation every 5 minutes. He does this because with mom he is the center of the universe. If he asks her to jump, she drops everything and asks how high! No exaggeration- she wipes his rear when he potties because he is too heavy to reach all the way around himself! She expects us to do the same! These little things are driving me crazy. Do I point out the bad behavior to dad knowing it will cause an argument between us because I am "criticizing the son"? (I have been accused of favoring the 16 year old. I don't think I consciously do but maybe...?) Do I ignore the fact he is morbidly obese for a 10 year old? I genuinely care about this little kid, but what is my role here?