John had dinner the other night with his only sibling's husband's mother-in-law's only daughter-in-law. With whom did he dine?

Let's trace this step-by-step from the end: The only daughter-in-law of John's sibling's husband's mother-in-law is John's wife. Starting from John and moving outward: John's sibling's husband = John's sibling's spouse (if sister, her husband); that person's mother-in-law = the sibling's mother-in-law; her only daughter-in-law = the spouse of the only son-in-law's wife = John's wife.