"Wives of the Prophet, you are not like other women. If you fear Allah, do not be too complaisant in your speech lest those with diseased hearts should covet you; but speak in a straightforward manner. And stay in your homes and do not go about displaying your allurements as in the former time of ignorance. Establish Prayer, give Zakah, and obey Allah and His Messenger. Allah only wishes to remove uncleanness from you, O members of the (Prophet's) household, and to purify you completely." -Surah Ahzab, verses 32-33
Prophet Muhammad's ﷺ family includes his wives, his children, people of his household (as mentioned in the Qur'aanic verse above) and his blood relatives who are believers.
Some of the prophet's ﷺ closest companions were also part of his family (via marital ties). For example, Abu Bakr As Siddeeq (RA) and 'Umar (RA) were both father-in-laws of the prophet; their daughters were wives of the prophet, Ayesha and Hafsa respectively. 'Uthmaan (RA) was a son-in-law of the prophet (he was first married to Ruqayyah, when she passed away he married Umm Kulthoom - both were daughters of the prophet).