Kid SIL - what would you think/say?

I love her but sometimes I get a little annoyed. There is a 20+ year age gap between my youngest sister in law and my husband. There is only 1 underage sibling living under the same roof, all other siblings have moved out.

She calls me pretty frequently to talk about little things. Well during our last visit my little sil asked my mil if she could have another baby. Well she said no, she can’t. (She’s in her late 40s & said she is DONE). A few days later she calls me... to ask if we can just pretend my baby is my mil baby. This annoyed me a bit, but I tried to remind myself that she is probably lonely.

I know kids say crazy things, but she asked again and I felt just as annoyed.

Tell me I’m being over dramatic or is it just a little weird to you too? I don’t want to pretend my baby is not my baby. I don’t want this to grow into something weird.