I live in an area where child abuse practically runs rampant. Many people won't step in and try to help a child who's obviously suffering because they're afraid to rock the boat, and maybe it would just be rude or un-neighborly. I, for one, couldn't care less about hassling someone, probably a total stranger, if there's innocent lives at risk. Your right to be "not bothered" doesn't supersede an innocent, helpless child's right to not starve to death.
As for the MIL's relationship with the baby's mother, there won't be much of a relationship to salvage if that baby ends up dying.
“Your task is not to seek for love, but merely to seek and find all the barriers within yourself that you have built against it.” Rumi