One's spouse's sister-in-law, especially one's husband's brother's wife, one's brother's wife in relation to the spouses of his siblings; either of two (or more) women who are married to brothers, in relation to the other.
One's brother-in-law or sister-in-law's sister; one's sibling's spouse's sister; either of two (or more) women whose siblings are married, in relation to the other.