The following are the most frequent entries for the relationship field.  However they are by no means the only choices and some unusual entries for this field can be found.  For example, there are 70 people in 1930 whose relationship to the head of household is "Br." (probably brother), one who is a "Br Father", and one who is a  "Night O Br" .  I can't even venture a guess as to what those latter two are.

head (used after 1880)
self (used in 1880)
husband, wife, son, daughter, father, mother, brother, sister,
grandson, granddaughter, grandchild, grandfather, grandmother,
stepfather, stepmother, stepson, stepdaughter, stepchild,
nephew, niece, uncle, aunt, cousin,
son-in-law, daughter-in-law, brother-in-law, sister-in-law, father-in-law, mother-in-law,
lodger, boarder, roomer,
servant, housekeeper, domestic, cook, employee, laborer,
teacher, superintendent, student, pupil,
patient, ward, nurse, convict, inmate, sailor, officer,