Non-nuclear futures

Al, my ex-wife (and mother of No 1 Son) has been helping out while I've been ill by picking the boys up from school on a Wednesday and taking them back to her place. They'll have dinner, play with Al's young one (he's 7) and Mac will collect them on the way back from work. Although, she might stay for a cup of tea and a chat.

Al and I separated a short time after No 1 Son came along - we were chalk and cheese, distressingly immiscible - but we've always kept close because of our son.

So, right about now, my boys are sitting down playing with their brother's brother and I'm sitting here wondering why we don't yet have a word for that relationship.