try killing the groom infront of the house, run back to the little place behind you, hide there, kill everyone that is coming to you, run into the house, go left to the greenhouse, go upstairs, step on the roof, where you're able to see the graveyard from, kill the father. done.

atleast that worked for me, even tho it's mass murdering. oh after you killed everyone there, change clothes to gang member.

For this to work bring your little suitcase.
when you arrive hang around the party for a while, notice the drunk guy, the one who can barely walk, follow him he goes towards the burned out house, there he will by his own devices become unconscious, take his clothes, no need to kill him.
Now go into the house through the front entrance, remember your a guest now, take a left when you enter the house, go to the rest rooms, theres an open window and a guard just outside, strangle him, but be sure noone sees you, take his clothes and drag him into the container which is just behind him, remember to close the lid. Now you can go to the second floor undisturbed take the unguarded door to the old mans room, where watches tv, be sure noone sees you. He can see you enter thats not a problem, plant a mine on his fireplace, go back out, take the stairs up to the attic, blow the fusebox, find the window with a view towards the pavillion, now wait for the wedding bell to sound, when the bride and groom are there, shoot the groom in his face, packup your rifle, check your map, be sure the old man is in his room, detonate the mine, now theres confusion and you can walk out through the back door undisturbed and sail away in the priest's boat.

When you start the mission, go into the big door infront of you, you will see a guy kissing some girl, kill them both with silenced gun. Get invitation card from them. (Watch out behind you there will come a guard, kill him also but he may not notice the other guards!)

Now you go into the house, go to the toilets (left side of the big stairs)

Go trough the window, wait till the guard goes sitting down on the chair (strangle him)
A fat man will notice the dead body, (this fat man needs to be killed, he's an objective!)

Anywayz now you can do 2 things:
Wait till the wedding starts, go behind the chapel or how is it called.. when they done kissing everyone walks away.. you have a clear shot to kill him and nobody will notice)

or

go back to the stairs @ the toilet stuff.. the groom will walk here a few times sometimes.. just strangle him, nobody will notice.. now run away to ya boat