Schwippschwägerin

Noun

 * 1) co-sister-in-law: the wife of the sibling of one's spouse (the sister-in-law of one's spouse), or the sister of the spouse of one's sibling
 * 2)  people who marry siblings, relative to each other: women whose husbands are brothers, etc.
 * 3)  people whose siblings are married to each other, relative to one another: women whose brother and sister are married, etc.