Cathy,
We have closer age gap (DS11 has felt threatened by DS9 in math ability for years). First, try doing homework separately. DH is a big believer in getting kids their own workspace in their rooms for homework (it's so much easier to use kitchen or dining room table, but make sure you've granted younger sibling computer time or TV so they're not together when you're working with the older one). I would Not copy the older one's homework for the younger one. If you want to challenge the younger one, get a workbook but make sure it looks different.
I have explained to DS11 that he has amazing talents that DS9 is not as good at and vice versa. I also told him that I thought the only thing holding him back in his math was himself, his mindset, his fears. He has definitely put forth greater effort and is doing well. BUT, he is not as naturally able as his younger brother when it comes to math. I think separation and time have just led to an acceptance of this and we move on. In other words, there's not necessarily a cure, just coping technique, then even well-wishing. I think even DS13 knows DS9 is more able, and he never worried about it (he thought it was cool).
I also agree with Kriston that the younger one learns to be sensitive to the older sibling--but this takes time and probably some battles & tears.
Good luck!
