A 12 year old girl and a 12 year old boy are two very different entities, maturity wise. I understand the other Mom not wanting to leave the twins with Fred to babysit. And with only 3 years between them, the twins might not listen to Fred. If they were 5, it might work OK. But at 12, I wonder if it is time to give Fred a trial run at staying home alone? Send the twins to the OP's, and leave instructions with Fred that he is to go to the OP's if he needs anything; otherwise, he's on his own.