I think Ursula is making a bit more sense.
If Rob only cares about himself and how the world fits around him; the same is also true of Ursula.
So she sides with Jess over Rob because she had a potential grandchild then dropped her like a stone when it wasn't.
The not turning up to dinner may have been genuine, rather than Rob making it up.
She's still an unnecessary character though and still doesn't explain why she's moved in.