Obsessed?

I may sound like the worst, but hear me out. My 15 year old step daughter is, to me, too obsessed with our two month old. She is trying to help, but she is going too far. She can't walk past without talking to and touching him, the second he cries shes picking him up, she hovers in his face constantly whether he's with someone else or on his own, wherever he is she has to be, she takes him from people, always has to hold him and feed him, she has to be the one to carry him and push him and put him in and out of the car seat and car. She says stuff that gets to me. Like my husband said he's gonna be a mommas boy and she says "or a sissys boy". He sees it as teasing and helping. I see it as obsession and trying to make my baby hers. She's forcing them to bond but I want that bond. Maybe it's cause he's my first. Am I being crazy? How do I tell her to back off without sounding mean and like I'm taking her brother away? Is it a phase? I hope I'm not being terrible but it bothers me a lot. Thank you ladies.