It's true that Rufus Sewell's brother, Jason Flemyng, is also an actor. However, they have not done a "brother act" together in the sense of performing as a duo.
Rufus Sewell is known for his roles in films like "The Sixth Sense," "Dark City," and "A Knight's Tale," while Jason Flemyng has appeared in films like "Lock, Stock and Two Smoking Barrels," "Snatch," and "X-Men: First Class." They are both successful actors in their own right.
While they haven't collaborated on a "brother act," it's possible they could in the future!