co-sister-in-law

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One's husband's brother's wife; or more generally one's spouse's sister-in-law, the sister of one spouse in relation to the siblings of the other spouse.
• noun 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).