You'll be attending a redneck wedding this time, where you'll have to kill the groom and the bride's father. Cool.
Turn right and go through the gap between the cottages. You should see a wasted party guest nearby. Wait for him to enter the building and go to sleep. Take his invitation and his revolver, then wear his clothes (now, this guy IS asleep, isn't he?). Enter the main building. You'll need a gangster outfit to feel comfortable in here. Grab one from the guard marked on the map below (*), sneak-sedating him. Get to him through the toilet. Throw his body into the small closet nearby. Now you can walk freely.
First we'll get rid of the bride's father. He's paying a visit to the family graveyard in the North-Western part of the map from time to time. Sneak on him when he's standing in front of the open grave and push him into it.
Get back to the mansion. Go to the kitchen and use the poison syringe on the cake. The groom will soon come here to have a taste - and that will be his end. Run fast, before someone finds his body. Get back your suit and escape the level using the appropriate spot near the starting position.