Advice???

Well not much has been happening..well interesting anyway. The only thing I was thinking about putting on here is wanting to know if anyone had any advice for me about my Kennedy. She is struggling with being the middle child and I am not sure how to help her right now. Kennedy is very much the alpha female anyway...but she is really struggling with something lately. She has started pulling her hair out!!! In chunks. I feel so bad. I just dont know what to do for her. Her behavior has also been worse then normal LOL. And she has been starting to be a bit mean to Kaitlynn. Pinching her and smacking the back of her head when she thinks no one is looking. Taking toys away and yelling at her. I want to help Kennedy, to make her feel better. But at the same time I have to protect Kaitlynn and I cant let Kennedy get away with being so mean. I feel like she needs to be disaplined for her wrong behavior, but dont know how to do it when it comes to what she has done if it involves the baby because that seems to be her sore spot anyway. Soooo UGH. I am kind of at a loss. So any advice would help.