It's an interesting question.
I see no point in an EP report on the other twin.
I'm not entirely sure whether an EP report on the twin in question would be relevant.
You need to be clear what it is you are appealing against. Are you appealing because, although your son has passed, the school is oversubscribed?
There was an interesting discussion about this issue recently:
www.elevenplusexams.co.uk/forum/11plus/ ... 4426#64426
I suggest you contact whoever is organising the appeal to ask what sort of evidence the panel might like. If they take the line "You are entitled to produce whatever evidence you wish", which is strictly correct, try to pin them down as to whether the panel are looking for an oversubscription case (see Q&As C2 for the sort of arguments usually put forward), or an academic case, or both!