Although the Torah forbids a man to marry his brother’s wife, if his brother dies without children, he has a mitzvah to marry the widow in a process known as yibum (25:5). The Gemara rules that after he does so, she becomes his full-fledged wife in every way. Specifically, she is no longer prohibited to him as his brother’s wife, and he must give her a get if he wishes to divorce her. Thus, even though the circumstances that created the yibum exception have passed, he is allowed to remarry her after their divorce as long as he is not a Kohen (Vayikra 21:7) and she has not married anyone else in the interim (Devorim 24:1-4). This subsequent marriage gives us a case of a man who permissibly marries his brother’s wife via chuppah and kiddushin. (R’ Ozer Alport)
Another case would be if the two brothers were geirim.
