When my twin brother married 6 years ago, he wore the family tartan, Graham of Mantieth. I was the best man, and there were of course groomsmen as well, we wore the tartan that the local shop had to rent out, it was close to the same tartan as my brothers but not exact, we all rented the rest of the outfit, the PC jacket and waistcoat, sealskin sporran, sgian dubh etc. The only things we needed to buy were the hose and tuxedo shirt. I think the best thing to do would be to go to the local tartan shop and see what they can do for you, as far as what tartan to wear, I would say whichever one your brother likes more.

Here is a photo of the family at my brothers wedding, I think the mix of tartans works well, it helped to determine who was who.