Bit of back story:
When I first got involved with my husband, 12 years ago, his two children from a previous relationship lived with their mother and spent one night a week and alternate w/ends with my husband.
My step-son carried on living with his Mum until he was 12 and then moved in with his Dad. I'd been due to move in that Autumn, but put it off to Xmas to give them some space and then moved in with them. My SS stayed with us until he was around 15/16 when he kind of slipped back into his Mum's household (along with his sister and occasional step-dad).
He's been off to Uni now and graduates this summer and he's decided that he wants to live with us again. His sister, my step-daughter, is happy as she finds him hard to live with (he's 2 years older and sits around in his pants playing video games and annoying her), and my daughter, his 8 yo little sister, is delighted that her big brother is coming back to live with us!
So, my main question here is, what ground rules would you suggest we set up to ensure that everybody is comfortable with the new living arrangement.