For the same reason, since behen is feminine, you use uski when referring to her, as well as conjugating verbs in the female form, since we are talking about a female, it is rehti, because verbs are conjugated according to the subject of the sentence, here "uski behen" which is feminine. So by the same token, "His brother lives in America" = उसका भाई अमेरिका में रहता है