Kids often don't understand exactly what it means to get a new sibling right away. And since babies sleep so much in the first month or two it takes time for them to see just how much time and attention a new baby takes away from them. My baby is 4 months old and my 2 year old son was totally fine at first. But he has started acting out a little bit. He's thrown a block at her head. Most of the time he's good with her, though.One of the best things you can do is to have him be your helper. Little kids love helping out in any way they can. Throwing away dirty diapers, bringing clean ones, finding blankets, pacifiers, or bottles when you need them, etc. Helping makes them feel special and appreciated. It makes them less like to have jealousy issues. Also, try to take your baby with you when you can. I know it seems silly if you're just going to the bathroom for a minute, but sometimes it's necessary. Chances are that once your daughter is sitting up, crawling, and really playing with toys your son will be more enthusiastic about her. Once they can play they can start to become friends.