I need an idea for a prank to retaliate for a prank that was pulled on my wife and her parents by her sister and boyfriend last year at Thanksgiving.
They thought it was funny to make everybody think they were going to have a baby together, and that one was already on the way.
I'm fishing for ideas here. Anyone has a particularly dastardly idea, throw it on me! 
Back in the day - when I worked for Bechtel at the nuke power plant in Richland, WA, a coworker (good beer drinking buddy) made posters of my head on the body of a "world renowned" accordion player that was playing in the area. He posted them EVERYWHERE in the 20 offices I worked in!
I had coworkers walking up to my desk saying, "I didn't know you played the accordion. I'll be there to watch your concert!
That dirty bastard! I can barely play a Jews Harp let alone an accordion.
Every town has one of those free small advert newspapers where you can cheaply advertise the private sale of your car, cat, used underwear, mother-in-law, and used stereo gear. In the Tri-Cities, WA where I lived, it was the Thrifty Nickle.
I bought an ad for an "All natural herbal colon-rectal cleansing service. We offer 8 different flavors including Jasmine, Basil, Sage, and many more. All natural, removes poisons from your body leaving you refreshed and empowered. All performed in the privacy of your home or in our discrete facility."
I added his name using his true first name of Mike but phonetically spelled his last name so as to change it. I added his home phone number as well.
The paper is released on Thursdays, that evening I gathered up about 25 copies of it, found the advert, circled it in red then took them to work with me the next day. I opened the paper to the circled advert page and dropped them throughout the break room and on tables and work areas throughout the office. He actually received about 10 calls at home inquiring about his "services".
It took him about two days to figure out who did this to him (ME!) and he never pranked me again yet I will say I love the guy and he is a great friend.
Early the next week the top-dog site manager for Bechtel, a vice president, shows up at my desk and in earshot of the 30 cubes in the office says, "that was the best joke/prank I have ever seen! DO NOT DO THAT AGAIN! As we do not want to make the wrong impression with our customer.