Census returns and other historical documents can be vague about family relationships. “Cousin” historically could refer a whole host of family ties, whilst “brother” or “daughter” on a census was often recorded rather than “brother-in-law” or “daughter-in-law”. Perhaps the census enumerator didn’t have enough space to record the full term, or perhaps the head of the household treated his daughter-in-law as his own daughter.
Another term that is less usually found in census returns is “good sister” or “good son” or similar, which also referred to an in-law, be it sister-in-law or son-in-law etc.
This example from the 1881 census records the head of the household, his wife, two sons and his “good sister”, or sister-in-law. It is a term to look out for in census records, as well as other genealogical sources.