Until you sell: How much will it cost to create the in-law suite and how much will you get for rent. Figure your return on investment conservatively.
Typically, in my area anyway, ROI is negative for this type of addition. Ask your realtor regarding resale in your area though.
Your question: "Is it worth putting in a small kitchen in a guest suite (over garage) for in-laws to use? Will we get our money back when it is time to sell? Right now there is a bedroom, 3/4 bath and sitting area." This is an excellent question as the market adjusts and families are getting closer together and sharing limited resources and residential spaces.
EMMY, before you do anything CHECK THE ZONING FIRST! I can't tell you how many homes I have been in where the owner has done exactly what you suggest and the Building Inspector has found out. This comes with a fine for doing work without permits and possibly a fine for making a single family home into a 2 family home. Usually a 2 family home is one that has 2 kitchens--at least that's the trigger, and it will increase your taxes a bit as well. Towns know that while it may be fine to allow a family to have 2 kitchens and call it a home with an in-law apartment, the family won't live there forever and then it becomes a 2 family house where non-related families can live.
Now, having said that, there are many ways--as they say--to skin a cat. The zoning will tell you what constitutes a 2nd kitchen. Normally it's a kitchen sink, and perhaps a stove with an oven. I have represented a number of builders who built homes with true apartments over the garage, and included a small under counter refrigerator (built-in) and a microwave (built-in), but no sink. There would be a large sink in the bathroom, just around the corner...and perhaps a sink in the laundry area. This qualified (in Newfields and Newmarket) as only a sitting area for the folks where they could keep sodas and make popcorn and have a little bar.
But I have also seen a house for sale in Kensington with all of the above but there had once been a kitchen sink as well and the seller was fined and the town caused the sink to be removed which made a mess of the counters and so forth.
Would you get your money back? My crystal ball is a little cloudy, but I would say at least you would get most of it back if not make a little on the effort. But your folks would get the value that you are willing to offer. So I'd say go for it. Oh and FYI, the tax assessor reads the MLS. So if a listing for a house says it has an in-law apartment, the assessor will check to see if the town has been getting it's full share of taxes.
Broker in NH