And, from a mom of twins, generally, it IS okay to only invite one twin....and most of my twin mommy friends would agree. Now, neighborhood play is different, but if your DS8 was allowed to take 1 friend somewhere, and he chose a twin...don't hesitate to ask one twin...let the twins' parents decide how to handle that (or address it with the parents first).
We have twins on our street that ds (8) is in school with and plays with. He has more in common with one twin than the other as they are very different. He has invited the twin he is closest to over for a sleepover w/o his brother. I did check first with their father and he said it was fine...they are different people! Ds plays with the one twin more, inviting just him usually, but sometimes both over to play. And just the one twin calls my ds over to their house to play. It seems to be working out just fine. I also have a ds5 who wants to play with all the big guys (he is in preschool and they are in 3rd grade!) , but I really try to avoid *making* them all play together b/c of the age difference. It's a tough break for the little guy, but we do the best we can. He can ride bikes or play in the yard when they are here, buyt ds8 can play with his friends alone too.

(it was a birthday gift). DS has also picked up some language - nothing really foul - but has picked up the word stupid and says butt all the time. Most often he uses the word stupid out of anger. Either by calling DH or I stupid when he's not getting his way. We have never called anyone stupid in front of him. And we very rarely say the word butt - usually we say bottom. I don't want him to pick up anything else.
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) to play at your house, but then only allow one child (when there is one that will be excluded)to play at theirs. Why should you be expected to host two of hers if she will only host one of yours.