When a new baby arrives the firstborns tend to develop feelings of jealousy.

Girl Jealous of Mother and Sister
1) Strengthen your connection with your firstborn
When your child believes that you couldn’t possibly love anyone else more than you love her, sibling rivalry melts away. Build one on one time with each child into your routine.
Connect with each child every morning and then again every hour you’re with them throughout the day by looking for opportunities for a warm smile, touch or comment.

Whatever your child says or does, try to see the situation from her point of view.
2) Get your child to laugh
Laughter helps humans let go of anxiety and transforms the body chemistry to reduce stress hormones and increase bonding hormones. It also helps children work through their fears.
When your child suddenly has to wait for your help and attention all the time, it’s natural for him to wonder if you’ll still be there for him if he really needs you.
Make him feel as if you are there for him. Encourage him to call for you when he is in need of you in anyway. As soon as your child calls you, come running, grab him up, kiss him all over, and toss him around. It’s a guaranteed way to get him laughing.
There are countless games that will get your young one laughing. It might be the best connection you’ve had with your child and he’ll be more cooperative for the rest of the day because of it.
3) Acknowledge your child’s mixed emotions
Every child is bound to have some complicated feelings about their siblings. It might be hard for you to hear about her anger at her sibling. But if she can talk about it then she won’t have to act it out. If she thinks her jealousy is unspeakable, shewill push it down out of consciousness. But stuffing emotions causes anxiety and rigidity and is unhealthy as they have to work hard to keep them down.
Such stuffed emotions don’t stay stuffed for long and then pop out at unexpected instances. And because they’re not under conscious control they often take the form of aggression, defiance, clinginess, or whining. To transform negative behavior, give your child some help with her “negative” emotions.

sibling jealousy1

Children get along best with their siblings when parents make it clear that all feelings are normal and acceptable, even while not all actions are permitted and civility is expected. Acknowledge how hard it is for your child, and give her permission to grieve. She’s lost something of value when a new sibling entered the picture. But as you connect with her, help her laugh and acknowledge her feelings, her hurt and loneliness will begin to heal. Your love and patience will give her relationship with her sibling a chance to blossom.
Be patient things will work out!