If one of your pawn's relatives/friends is in a non-enemy faction, he'll invite a said pawn(s) to come to his marriage. If the player lets the pawn go, he will get a mood buff "attended friend's/son's/whoever's wedding" and some relation bonus for his relative/friend ([relative] attended to my wedding!) and a smaller bonus for his/her future wife/husband (my fiancèè's relative came to my wedding (or no bonus at all for the other guy)) and faction they belong to. If, however he doesn't go he'll get a small relation debuff with both of them (relative didn't attend wedding). If the player decides to also get some gifts for them (silver or sculptures or anything) the relation boost will be higher depending on the value of those items (relative brought expensive/cheap/good/excellent gifts on my wedding!)