उनकी means "their", whereas उसकी means "his/her".
However, if you want to show respect to the "he" in "he as a daughter", the formalization makes it plural and you would use उनकी.
उसकी एक बेटी है = he/she has a daughter
उनकी एक बेटी है = he/she (formal) has a daughter
उनकी एक बेटी है = they have a daughter
She's a daughter is not the answer, and neither is she is a daughter. Word-by-word this is the translation: उनकी or o-na-ki, means "They", एक or ek, means "one", एक or bae-ti, means daughter, है or hai, means have, so it would translate to "They have a daughter."(as it says above) taking into account Hindi grammar rules.
I'd really like to understand this too. I think uske/uski is "he/she" while unke/unki is "they". But if someone could clarify the difference, it would be truly helpful