It sounds to me like you may need to find your own place, I don't know about where you are but here there are resources for mother's with children that could possibly even buy you a home. I know that they would pay rent.
You have too much negativity around you. Your mother is putting you down constantly and you have no time to yourself. So I think that it is time for you to find your own home so that you and your children can be comfortable. The situation there sounds like it is only going to get worse with all the tension from the gambling of your step dad and the way your mother is in denial about it. She will constantly take it out on you.
You said that you were getting counseling for the children, well, what about yourself? I think that it would do you good and hopefully they help to find you some resources. You being at your mom's is only enabling your stepfather to gamble since he has no bills to pay.
I am sure that you are a wonderful mother, so don't blame yourself. And I am sure that you were a good wife to your husband, so don't let his demeaning words make you feel otherwise. You don't deserve all of this negativity.
So see about the counseling and see about finding a better place to live where you don't have to be treated badly. You do not deserve that.