Hello I have a wonderful relationship with a woman. We have been together for 12 months.

Her ex-lover now has a wife and child on the way. They recently moved to my girlfriend's neighborhood. My girlfriend is now great friends with her ex-lover's pregnant wife.

My concern is that she might help take care (baby-sit) their child once it's born.
It makes me feel weird if she were to take care of her exe's child. It means she is too close and tight with her ex. It also means she would project "mother" energy to her exe's child which is not mine. It feeds my sub-conscious archetypes. I feel like historically, in human history, when your girlfriend takes care of her exes child, it ain't a good thing.

I brought up this concern with her last night, and she had trouble understanding why that would bother me. She didn't see what the big deal was.
I said it was very important to me, indeed I have never stopped her from doing anything, but this is one thing that would eat me emotionally.
Ultimately she says I am more important than her friend, so she will respect my request.

So my question is:

I am wrong to feel weird about my girlfriend potentially helping take care of her exe's child?

Do you think historical archetypes confirm my unease?