We are setting up a double wide on our property as a mother-in-law house. We want to supply her house with water from our system, which is a residential well. Her house has it's own pressure tank. Where do we tie that into our system before or after our pressure switch?

Her new double-wide has a pressure tank? That's a new one.

At any rate, I am assuming that the well is on your place. You will run the line to mom's after the switch. I would put a checkvalve on the line running to mom's. That will prevent your water use running her pressure up and down.

Thanks jlisenbe. With the checkvalve in her line what will prevent her water usage from running our pressure up and down?

Basically, nothing. You can only run the checkvalve in one direction. I am assuming that you will be the primary wateruser and, thus, would be the "chief offender" in affecting the other person's pressure. Honestly, I would tend to move her tank to my property, hook the two tanks together, and simply run a line to her house. That will give you twice the capacity and make your pump run more efficiently. You could also eliminate the checkvalve. However, it would not solve the problem of pressure swings, and it would raise the possibility of both houses using water at the same time and exceeding your pump's capacity to supply water.