Use a hyphen to link a relationship term: sister-in-law, mother-in-law, father-in-law, brother-in-law, ... Plurals: sisters-in-law, brothers-in-law, mothers-in-law… But: stepmother, stepfather, stepdaughter, stepson, stepchild.

Is step daughter one or two words?

Word forms: stepdaughters

Someone's stepdaughter is a daughter that was born to their husband or wife during a previous relationship.

Is there a hyphen in step dad?

According to Collins you can use both, although they list it with the spelling "stepfather".