M.M.
I had this problem with my two sons. My oldest was three when his brother was born. He did not regress. But he was very ruff with him and it was to the point where I was terrified to leave him alone with his brother. He would body slam his brother in the stroller when I was not looking. (He would run up to it and just lay on his brother really hard.) When in the swing he would go up to and swing it really super fast or hold the swing up way to high. It got so bad that I went through postpartum depression. They are now 7 and 10 and they fight a lot and His brother to this day is still to ruff with his brother a lot of the times. When he was young I would punish him give him time outs nothing seemed to help. My advice is get a handle on it now or get some counseling or it will be that way for a very long time. I have a third child and both children are very loving toward him so it totally jealous thing.