All children get a little jealous when a new baby comes along and for moms that use the adage terrible 2's, for our family is was the 3's.
But I think this needs to be address with time outs.
This is what I would do.
When you see her doing things she is not suppose to do, first come down to her level (eye to eye) warn her and explain what she is doing wrong and about timeout.
Second, if she does it again, come down to her level again and say do you know what you are doing wrong? IF she says yes, then ask her do you remember what I told you about timeout? If she says yes then tell her you will need to go into time out for a short time to think about what you have do. If she says, no, then explain it one last time to her but do not put her in timeout.
Once she has been in timeout for about 3 minutes. Walk over to where you have her in time out (Mat or chair) come down to her level and ask her if she is ready to >>>what ever the problem was<<<<. If she says no, then say well maybe a little longer in time out might help and repeat. If she say yes, then ask are you sorry did that to your brother? She should say yes, then give her a hug and say I Love You and let her return to what she was playing with or to eating. It is very important to follow through with timeout and don't go by what some of the books say about amount of time. Timeout need to be looked at a punishment and 1 minute in timeout is not long enough. I say it should be a minimum of 3 minutes. Also make sure when she is in timeout that you pay her no attention unless she leaves the timeout area.
Another way you can get bad behavior to good behavior is through a sticker chart. Everytime she plays good with her brother or eats her food without touching her brothers or maybe takes her dishes to the sink or helps make her bed, etc. She gets a sticker on the chart and when she gets so many stickers she gets a special treat. It can be her favorite food for supper or an ice cream cone or something special from the $$ store. Another way is to make a magnet chart. Buy a magnetic board and draw squares for each magnet and when the chart is full time for a reward. You can use make your own magnetic stickers or just buy some at the $$ store. The one thing good about the magnetic sticker board is that stickers can be taken off when there is bad behavior.
I hope some of this helps. Good Luck..
God Bless,
S.