You need to find some common ground when teaching/working with your brother.

What I mean is that if your brother has been skiing he would understand that balance from the feet/knees/hips all work in harmony and are as one and if one side should break down the other will try and make up the difference, now if he is swinging/moving to fast he will fall over simple as that.

Regarding the left leg bending to much don’t worry about that better it bends than not at all.

This is a good guide to balance, try and get your brother to set up over the ball (that’s before he starts moving the club away) at that point ask him to stay in position and take you butt end of the golf club and from his front just gently push him in the chest then go to the side and push him in the shoulder do this all the way round, now if he does not fall to one side and has a good posture he will resist the pushing by you. If he fails at this point you will need to rework his set up (do it from ground up to his head)

Now ask him to swing half way back and then do the same with your butt end of the golf club, start from the front and work your way round his body go in a clockwise direction. At this point he might fail (possibly when you push his right side) due to the lack of control in his left leg, I would also think he not transferred to much weight over to his right leg at this point either.

Now ask him to swing as far back before he starts any downswing (Some folks think it should go right to the top and parallel to target, that is not always the case. Some golfers can’t get there, me for one lol I’m way to old)

Do the pushing again all the way round, and check for balance, if he should topple one way (you will need to look for a remedy if that is the case) ask him to start over and get to the top.

Now if he is not too bruised at this point! He/you will have some idea at what point in the swing things start to happen, like weight transfer and balance.

This is only a guide and the pushing should only be a small amount, just enough to let him feel resistance in his set up.

Regards Cliff