"Su", like tu, can mean "your." The difference between your (tu) and your (su) lies in the degree of formality the speaker wishes to convey. Mi casa es tu casa. (speaking to someone you would address as "tú") Mi casa es su casa. (speaking to someone you would address as "usted") Note: The two words "tu" and "tú" are pronounced the same. Tú (with the written accent) is the subject pronoun meaning "you" (informal). Tu (without the written accent) is the possessive adjective meaning "your" (informal). Su has four meanings: his, her, their and your (formal). María busca a su hermana. María is looking for her sister. Juan busca a su hermana. Juan is looking for his sister. Ellos buscan a su hermana. They are looking for their sister. Su madre busca a su hermana. Your mother is looking for your sister. If the meaning of su is not clear from the context of the sentence, a prepositional phrase is used in place of su. María busca a la hermana de él. María looks for his sister. El hombre busca las llaves de ella. The man looks for her keys. María busca el cuaderno de Juan. María looks for Juan's notebook. El hombre busca las llaves de Samanta. The man looks for Samanta's keys.

La frase está perfecta. The phrase is perfect. Un saludo

La frase está correcta, aunque también puedes decir "¿su madre está de acuerdo?" o "¿está de acuerdo su madre?" The phrase is correct, but you also can say "¿su madre está de acuerdo?" or "¿está de acuerdo su madre?"