"She refuses to put him on his back alone to sleep" is the safety issue, I think, but there's nothing inherently unsafe about an infant sleeping while held by an adult, as long as the adult is also awake, so I'm really confused.

"They seem to completely ignore the fact that I’m breast-feeding him" so they... feed him formula instead of the pumped breast milk you provide? Give him solid food way before he's ready for it? This letter is really remarkably vague about what the actual childcare issue is.

However, "she... doesn’t seem to want to have anything to do with us" sounds quite hurtful and like it's probably the core issue here.