Some people go through their lives following the social rules of monogamy. Some people don't. Often times people from group A are married to members of group B.

Some relationships can tolerate this behavior, if the partner knows about it. Many don't. Sometimes the behavior is out of control and needs professional attention, because it has more to do with self-esteem and emotional problems than it has to do with the need for sex in and of itself. Some marriages are unhappy. Some marriages really are "open" and hopefully both partners practice safe sex when they're with others.

If you are omniscient regarding all aspects of your brother's relationship and are in a position to sit in judgement on his behavior and that of his spouse and his second partner, then by all means go ahead and make your pronouncements as suggested above. For me, all I can suggest is that you

mind your own business

.