Levirate marriage makes no exceptions for polygamy.
I believe this means where a younger brother was responsible for wedding (and then bedding) the widow of an older brother who died childless, so as to attempt to father children to the name of the older brother.
And if little brother had a wife... now he has more....
It wasn’t ideal, but neither was it considered an unalloyed evil.
Big problem in arrangements like this is, of course, jealousy!