When we first went "family camping" with other families, we used a big cabin tent that we bought from an online discount place (Sportsman's Guide). It worked for us for 3 years, which helped in our decision to buy our own camper.
One of the other families we camped with those first years had a nice 24 foot TT. I asked him the first time we went out about using the toilet and shower in his TT, and his immediate response was "No". He added that he has to drain the black and grey tanks and keep everything clean... but he was adamant that basically only his own family go inside the camper. I was a little put off by this, but let it slide.
I did realize that one of the best things about a camper? The awning! Great to have the area for shade and sitting under in rainstorms.
A few years later when we bought our own TT, I understood his attitude.
So, you have to put your foot down, and say "No". I was in a similar situation with my brother-in-law, and we were adamant about our camper being off limits - I am sure he probably thought we were being stuck up snobs. You could welcome your brother and family to get their own campsite for a tent, but your camper is off limits. The conversation with your brother is going to not be great, try to be polite, but it will have to be taken care of. Good luck!