Wet nurse
Far back in history, rich mothers would often employ poorer women to breastfeed their babies alongside their own. It was a fairly normal practice, as all breastmilk is essentially the same and just as nutritious, as long as diets are reasonable.
It’s not really the done thing anymore, though, and I can understand this mom being a little weirded out by the discovery that her sitter was nursing the baby while she was looking after him. Aside from it being unusual, the mom had probably worked hard to leave expressed milk for her son to drink.