You are such a caring and giving person so it's understandable that the problems your sister and brother-in-law are living with is breaking your heart. But you said it yourself - there really is nothing you can do to change your sister.
By your own statement, you have spent hours on the phone with her and talking with her. Still, she does not acknowledge the frustrations of her husband nor does she see a problem with her own resistance to counseling. I wonder if her apprehension to talk to a counselor is due to something that goes much deeper than the issue of caring for your mom. Who knows. We never really know what motivates someone to be a certain way.
As hard as it is sometimes Mimi, we have to remain silent and just be there for someone and to just listen. It isn't in our power to change another person's ideas or beliefs. Unless you feel that your sister is open to suggestions and is willing to take the first step toward saving her relationship with her husband, there really isn't anything you can do, sadly. Some people just don't see beyond the end of their nose about certain issues.
I know you love your sister and I do recall how she was completely oblivious to your frustration when you were caring for your mom last year.
You have a tremendous capacity for love and caring for your family and dear friends. It shows in your writing and sharing on this site. Those of us who visit this board are so lucky and blessed to have you here to share your thoughts and knowledge about the strength, courage, fear, pain and joy of being a caregiver.
As for helping your sister understand all of this, you may not be able to do anything to convince her otherwise. However, continuing to be there and listen also gives you the opportunity to give her food for thought. Knowing the little I do about you and your own journey as a caregiver, I am certain you will continue to be there for her.
Hang in there, Mimi. You are an angel.