Hi, I can totally understand your worry. Feed your son in a different place as the cousins. As for me, I will totally avoid letting my son see what they are drinking(soda).
Your son is still young, educate/explain/talk to him on the importance of eating a
variety of food. It will be better if you could educate your son in the presence of the cousins and hope the cousins will improve on their eating habits.
Tracy Ann Agay