Not action adventure based, but Pillars of the Earth has an excellent sibling rivalry between stepbrothers. Much fighting. Much angst. And much "you stole my girl!"
Anansi Boys by Neil Gaiman pretty much focuses on this. Its about two brothers who are the sons of the spider god, Ananzi, who is also the inspiration of Brer Rabbit. The one brother has good looks, charm and god powers while the other is slightly overweight, awkward and as normal as they come. The handsome one starts interfering in the other one's life and the handsome one falls for the awkward one's girlfriend.
