So does Dean, actually, because he’s not sure Cas would. They both invite her without telling the other.
See, more than anything, Dean wants Castiel and his mom to reconcile. Because it is so unfair that Cas has a mother– a real, living one who cares about him and loves him– and they won’t even talk to each other and their relationship has been bad since before Dean came into the picture.
He doesn’t know why, but he hopes that her coming to the wedding will force a reconciliation.