I thought that was her being maternal, "but PETER'S HERE, how could you invite his ex?" Though if that's the case, it's a strange way of putting it. And if it really would have bothered Peter that much, that makes Anna look a bit insensitive. Maybe May was just being having a flashback to an earlier characterization and being overprotective. "Oh well, as long as she's here, to the reception. Wheatcakes for everyone."