I would view it as Anna is setting up her former nurse, who she must have come to like a great deal, with someone who she thinks that nurse will click with. She probably doesn't know a whole lot of guys to match make with and as far as she's concerned Peter is a good guy.
And back in those old days of conspiring, I'm pretty sure she knew her niece was "fast" and was probably hoping a hook up with Peter would slow her down. When it comes to this current breakup, without knowing the reasons, she probably thinks it's MJ who doesn't want the relationship at the moment (and she would be right).
Plenty of folks stay cool with the family members of their ex. Heck, some may even date!
It's really not that strange.