One's brother-in-law or sister-in-law's sister; that is, one's sibling's spouse's sister (one's sister's husband's sister or one's brother's wife's sister). (in the plural).
(uncommon) One's wife's sister's husband; or more generally one's spouse's brother-in-law, the brother of one spouse in relation to the siblings of the other spouse.
There is no common word (though see below) for this relationship in English, but there is one in Spanish: concuñado/a. So William is your concuñado. This word.