Please I NEED your advice!!

I'm not a native English speaker so sorry in advance.

So my boyfriend has a cousin? (his brother's daughter) who I cannot stand at all!! She's 6 yo and probably has a severe case of ADHD. When she sees me she locks her hands around my belly and just hangs there so I literally have to drag her around if I want to move 😐 she keeps saying "little boy, little boy" and denies to let go of my bump and sometimes rubs her face in my bump and my clothes and just aaaaahh!!!! I get mad just writing this, she's completely obsessed!

I have two sisters, one is 8 yo and the other is 4. I've told my boyfriend that I don't want any of them to come visit at least 3 weeks after the baby's born (and especially not his causin, but I don't tell him that) but he doesn't really seem to agree on that and everytime I bring it up he says "yeah we'll see"...

Now I'm most worried he won't tell his brother that no kids are welcome at first and his brother will just show up with his obsessive daughter and I have no idea how to react or what to do if that happens.

This is literally my biggest worry, I hope you understand how much I can't stand this girl and how I DO NOT want her around my baby for some time. Can you imagine how she will be when he's born 🤤😣😣😣

please help me figure out how to react to this situation