Quick poll, ds is friends with a boy whose twins in his form. Ds is not friends with the twin brother who's in his form but they are friendly enough. He really gets on with the twin who's in another form. all kids in Y7. Ds wants to invite his friend who is not in his from but not the twin who's in his form. Is he being unreasonable? Will the twin in his form feel annoyed at being left out?
YABU you can't invite just the twin from the other form even though he is ds' mate without also inviting twin who goes to his form as he would feel left out.
YANBU it's ok to just invite the twin who is Ds's friend even though they are not in the same form together