I have sole custody and dad has no visitations! But he demands to speak with them and see them?

No.

Get this down in writing for him.

Write him a posted letter and explain to him that:

1)  He has failed to pay required child support dictated by court order.  And so he has broken the law.

2)  You have asked the children if they want to go over and see him and their grandparents or talk to them on the phone and they have declined...however, he is welcome to sit down and write them a letter.  They will answer if they choose.

If visitation was not required as part of the divorce, then the unwillingness of your children to communicate with their Dad is this choice and it should be honored.

maybe he can get supervised visitation

He has no rights until and unless the Court awards rights to him.  No, and, in fact, if he has no rights and you give him rights you COULD make it difficult to go back to "no rights."  

Child support and visitation are NOT related - verify with your attorney.

YOU may have opened the door to a revised Order by allowing him to spend time with the children.  It's just that simple.

You make this kind of tough when you don't give your kids' ages.  There's a big difference between 5 and 15.  One of the reasons I mention this is if there's supervision when they're with their dad, the younger they are the more important it is that you make this decision and don't let them beg or negotiate.  Kids aren't always the best judge of what they need. 

Also, you want to make very sure they aren't picking up on your feelings about him (or your husband's).  Never forget that you are the most important person in the world to them.  If they overhear something or even just read your body language, they might subconsciously be trying to "please you" by taking your side over their dad's.  This is actually fairly common. 

With this said, the exception is if you ever feel there's risk in going over there.  You didn't say anything about what caused his bro's kids to end up in the system, but this is never about something minor.  If the bro moves back in or something like that happens, you'd want to adjust to that.

" Do I have to force them to make calls and see him?"

Not unless a court tell you to, if you are not under a court order you two would have to agree to such a meeting in front of a judge.

" The kids have told me many times they don't want to talk to him nor even go to see him they rather stay home. "

Make sure that remains their decision and not yours.   If you make it for them they'll remember you as the one who created distance between them and their father.  My mom let me see 'dad', she knew he'd be a letdown and figured better for them to see it for themselves, and she trusted we would,