Definite spoilers approaching, a step by step account to completing the mission......
First of all, don't enter the mansion when the other 2 guests arrive. Go around to the right, and you'll see a guard patrolling on his own. Follow him to the very end where he looks out over the edge, sneak up behind him, and use the sedative needle on him. Steal his clothes and his shotgun, and pick up the ammo from the stool close by.
Turn back around and start walking back towards where you came, and you'll see a door on your right. Enter it, then enter the door on your left. You'll be by a building with some tables outside it. Go inside this room and there should be a guard right in front of you. You'll also notice a set of crates on a pully system. Go down the stairs and stand under the crates, then make a note on the map of exactly where you are. Go back up the top of the stairs, wait for the guard to go downstairs, and place a mine on the pully system, and leave the area. Go back through the door which you came through a minute ago (to the right of the building you were just in), stand in a corner, and bring up your map. Change the map location to the building you were just in (I forgot what it's called but you'll know). Wait a while, and you'll see your target walk around, and eventually go in the area underneath the crates. Blow the bomb using the detonator, and you'll kill him.
Now go back towards the building but go past it, towards the far right corner of the courtyard (there's an exclaimation point on the map about this). You'll notice there is a drainpipe you can climb up, so do it, get on the roof, and immediately climb in through the window to the right of you. You're now in the room with the father who needs to be killed. You can do this in one of two ways. Wait until he is playing his instrument, and kill him from behind in any manner you choose, or wait until he's finished, follow him into the next room (there's a guard there asleep, so make sure you're creeping), then push him over the edge of the balcony.
Now creep behind the sleeping guard, use the sedative on him, and take his clothes and his gun. You can now walk out the front door of the building with no problems. Make your way down to the jetty where the plane is, and you're done.
Mission over, two people killed, not a single shot fired
