how to seduce my married brother
If you suspect your husband's sibling is trying to seeuce him, you just might be right. This is more proof.
OK, much of what I wrote here applies.
That the brother is married matters if it is a closed marriage. This blog does not advise cheating, and where consanguinamory is still illegal, the bother's spouse(s) can use the law to destroy the brother and his sibling. The whole thing can be an ugly mess, even if law enforcement isn't involved.
However, if marriage is open or if it is dead, and those are two very different situations, there is a chance that might be right to pursue. In some open marriages, the brother's spouse can even be the key, for more than one reason. The spouse can test his possible interest, the spouse can set up certain situations. Use your imagination here.
So much depends not only on the conditions of the marriage, but the personalities of the people involved.
People are always welcome to contact me via private message or email (see my instructions on commenting) to get my specific perspective about a given situation.