My sister and her husband (her second husband) have a son in primary school and they let him get away with all sorts of things that they would never let her older son from her first marriage do.
When they first got married her new husband treated the older boy as if he was his own, but things changed when the new baby came along.
The older boy has noticed the difference in the way they are treating the younger one and he is angry about it and this shows in the way he has begun to behave.
My sister and her husband say they treat him more strictly because they are afraid he will turn out to be like his father and the way he is behaving proves that they are right.
Queenie, what can I do to help him?—Worried aunt
You can try explaining to your sister and her husband that if your older nephew turns out to be like his (apparently not so nice) biological father it will not be something passed on to him from his bio-dad, but because of the way they are treating him. However, they may not be willing to listen to you.
Try to persuade them to get professional counselling for the boy, and/or family counselling for all of them. The parents might be willing to listen to a professional counsellor, and at least it would help your nephew learn to deal with the situation.