All you need is a pressure reducing valve at the line to the barn, and another at the line to your house. This will reduce the pressure to whatever you want.
The main problem is that your inlaws well only makes 6-8 GPM. A 200' deep well, with water standing at 80', has about 120 gallons of storage that you can use at any flow rate, before the well pulls down and all you have left is 6-8 GPM. At this point, if both houses were using 5 GPM each, you would pump the well dry and the pump would shut off. Depending on how and when both houses use water, this could be sufficient by itself. I would ditch a line, hook up a couple of pressure reducing valves, and see how it works.
If you then decide you need more volume of water, you can put a 1,000 gallon storage tank at your house, and fill it with the line you ditched in. Then put a booster pump and controls in the storage tank to supply your house. You can restrict the line to the storage tank to 1 or 2 GPM, so your can refill the tank anytime without affecting your inlaws water supply. Or you can put a timer so that your storage tank is only refilled at 3 in the morning, when the inlaws wouldn't see it happening.
A well that only makes 8 GPM will make 11,500 gallons in a day cause there is 1440 minutes in a day. This is enough water to supply 30 homes but, you have to use storage tanks and booster pumps to be able to supply the volume of water needed for short periods of time.
PS; use 1.5" or larger poly line to the house so you don't have friction loss problems.