She is treating her like a visitor-a shame that she can't just treat her like family.
She IS a visitor.
And visitor and family are not mutually exclusive groups.
My mother is family, but when she visits my home she is a visitor. This is not where she lives.
What does it mean to treat someone "like family"?
Do you mean she should treat her like her grandmother? Great Auntie Nellie? Like her father? Like her brother?
"family" is a pretty vague concept when it comes to "treating someone like family".
It depends on the family, surely?
And that's the thing about in-law relationships, you are suddenly officially "family" without having grown up with the same expectations, traditions, idiosyncrasies, eccentricities, rows, boundaries, notions of privacy, sense of decorum.
In-laws come to feel like family gradually, over many years. Demanding that woman who had a baby 3 weeks ago treat a woman who is in her home making her feel uncomfortable "as family" shows no appreciation at all of the complexities of human feelings and relationships.