Mamma66 can I ask a couple of questions. Where is your stepsons' mother? Is there any chance she could share the weekends and your SS go there and be supervised?
Why does your SS need supervision? Has he shown signs of aggression or other behaviour that has required social services to intervene?
The key to this could be your DH. It seems your SS is behaving irresponsibly. Not pulling his weight with his children or work. But without knowing his background it's difficult to say what is to blame. If it's sheer idleness and he is going out or on computer games or watching football while you entertain/feed/bathe the children, then your DH needs to sit him down and give him a good talking to.
I had a similar, but not quite the same, situation until recently. My eldest sons' relationship broke up so he came back to live here. My other 2 sons were still living here. My 2 GDs lived with us part time and sons' new girlfriend here (I didn't mind as she was so good entertaining the children). Every weekend from fri to sun and school holidays. I was having cancer treatment at the time (chemo/radio/4 operations) and the catering/laundry/general chaos was a nightmare. I still don't know how I got through it to be honest. But, there was one very important difference, my son was with them all of the time, a great father, entertained and took them out swimming, cinema, park, cycling. He didn't take advantage, worked full time, took his holidays during the school holidays. It was just difficult but we persevered and of course, it did end. He moved out last year, girls only come here on sundays now, we are all so close knit it's lovely and the girls are doing really well.
So, you may be able to do some things now to improve the situation but do be careful about throwing him out as he will probably be fine but the children may suffer the insecurity of having to visit a day centre with a social worker present or ex DIL stop all contact if she has reason not to trust him.
As for your brother, you've explained. If he is a bit off with you tell him you'll help him out if he comes over and looks after everyone while you and DH have a weekend away. That should sort him out