I would tell her it’s stressful enough for new parents and her anxiety and fears are just adding unnecessary difficulty to the situation.
Could you sit down with her snd ask her why she is so anxious. How was it for her when she had her son etc. and does she think she has got more negative/ afraid as she has got older. I find if I can get someone like that to talk it can remove some of the difficulty.
It is quite normal these days for new parents to not have their own parents so involved. Maybe she can’t handle that and this is her way of trying to control the situation so she doesn’t need to feel redundant.