I will assume your father-in-law has minimal experience. If he has been out a while, so be it.
I find it best for a new rider to practice side-hilling on flat ground. Get them on the left side of the sled with their correct foot forward (left), and the incorrect foot behind (right). Now lean the right knee into the seat so you have in effect 3 points of contact on the sled without hanging on. At very slow speeds, <5 mph, lean away from the sled and turn away (right) and get the sled to start to lean up on its side. I'm thinking the sled may only be at a 20-30° angle, but for an inexperienced rider it can feel intimidating. Now the rider can start steering by turning, (sounds simple, but the natural tendency is to let the sled set back down) as the sled will tend to veer to the left. If the rider can practice getting the sled up on one ski and riding for a distance, then have them try the other ski. I believe this helps them build confidence that they can control/guide the sled at an angle other than flat.
Next is my theory and I'm sticking to it.
Picture a long skewer running through the length of the sled entering and leaving at the mid point of each bumper. Assume the roll center is exactly at this point so that if the sled was picked up by this skewer it could easily be spun like a pig on a spit and not have a heavy side. Now, picture your sled on a treadmill with the skewer anchored. To get the sled to roll to the left, you need to turn right, to in a sense, roll the sled underneath the roll center (skewer) and bringing the handlebars closer to you (again assuming you are on the left side of the machine). If you were on the right side, you would steer left to roll the sled under its roll center again. This action of turning away from the direction you really intend to go is called the counter steer. I also believe that as speed increases, you don't need to add as much counter steer input to make the machine respond. I think this is where beginners get uncomfortable again because the machine responds so quickly to too much input, and catches them off guard. Now as the rider gets more saddle time, they will find that more counter steering may be required as the side slope increases (beyond the 20-30° angle that they got comfortable with on the flat ground).
As they get experience and take on steeper or longer runs, they will have to learn where to place their weight on the boards. On steeper runs, the rider should be watching the sled angle in correlation to travel direction. If they are too far back on the board, the rear of the sled can wash out and not be very responsive. If the rider is too far forward, the inside ski may bury too deep and tend to roll the sled into the hill more than necessary and therefore start to loose traction/ground speed.
Now, at the end of a good ride, have them show you how well they can ride on one ski on the flat. I believe these two ideas are related.