Your sister-in-law is by definition one of two things:
- She’s your spouse’s sibling.
- She’s your sibling’s spouse.
In the first case, your sister-in-law’s brother is your brother-in-law, because in law the couple becomes as one person and he is your own spouse’s brother. However, in the second case, that sort of sister-in-law’s brother is not your own in-law.
But it really doesn’t matter either way. That’s because in neither case are you related to this gentleman by blood, and only in the first case by marriage. Since there is no blood relation, there can be no genetic argument against either of them even if you should be female.
Indeed it is perfectly common for one pair of siblings to marry another pair of siblings, provided that neither pair is already related to the other pair by blood.
Any issue from those two unions will be double-cousins to each other, so twice as related as a normal pair of cousins, the same has half-siblings. (Unless the pairs are identical twins; then the cousins are equivalent to full siblings.)
As for what is right and wrong here, that’s merely a cultural matter, and the answer varies according to which region you live in. Cousin marriage is extremely common worldwide, for example.
You are not related to your cousin’s wife’s brother unless you are already related to those two siblings via some other relationship that has nothing to do with your cousin’s marriage.