I found involving them when I was feeding or changing baby helped. TBH, the worst worst spot of jealousy I have ever experienced was when Lyta was 4yo. She tried to stop Harry from crying so it wouldn't take my attention away from doing something for her. I walked into the dining room with her sandwich and found her with her hand over his mouth and pinching his nose. It was so so hard not to fly right off the handle with her in case it made the situation worse, but she really had to know what she did was serious.

After that I kept him with me on a sling and had them help me at feeding time or to change a nappy - that way they felt involved.

Good Luck!