You are struggling with the dilemma that every parent experiences sooner rather than later, namely when to let go of your child.
We know our children will in our hearts always be our children, and we will do anything to keep them away from hurtful things. However, a wise parent lays a good and strong foundation, and then trusts a child to build on that foundation.
Your daughter is an adult, and she should make her own decisions. You know already that she is able to make good decisions that are right for her, and you know how to support her in those decisions.
If she chooses to return to her husband, she will do so as a wiser, more mature woman, knowing what she is getting herself into and knowing what to expect. If she chooses to not return to her husband, she will also know what impact it will have on you, and how her life will change as a result of such a decision.
You obviously love your daughter dearly and you want to see her happy. But look back at your life, and you will realize that life sometimes requires us to go through painful experiences so that we can have the reward of happiness. The same holds for your son-in-law. He wants to be happily married and keep his parents happy. He will also have to make some choices and you cannot make those choices on his behalf. You can only have faith that everything will work out perfectly for your lovely daughter.
It is not easy to let go, but your daughter is an adult, and the best you can do is support her, whatever her decisions are.
No matter what happens, you will always know in your heart that you are a good mother and that you have done everything you could for your daughter.
Your daughter will make her own choices and sadly, so will he husband. You cannot take responsibility for either of them. If he loves her enough, he will find a different way to take care of his parents and live separately with his wife.If he does not love her enough to leave his parents, rather they separate now than they live a life of irritation and unhappiness.
I know it does not provide you with a solution, but trust me when I say that everything will work out perfectly.