Fist throwing at 18 months old

Lia

My 18 month old son hits me in the face, he hits my younger sisters really bad sometimes to the point that they cry. He does this sometimes out the blue or when he doesn’t get his own way. I’m currently 20 weeks pregnant and I’m really scared that he’ll hurt the baby. Especially because he is so territorial. Has anyone ever experienced their oldest child being really rough with everyone except their younger sibling?