Is it obligatory to inform one's family and fiancé about repentance from fornication that occurred before marriage?

You are not required to tell your family or your fiancé about your falling into zina (fornication/adultery). Rather, you must conceal your sin. However, if the fiancé stipulates virginity and it is no longer present, you must inform him of the truth without lying, using allusions (ma'areedh). For example, you could mention that its absence was due to menstruation, a fall, or riding on something sharp. And you must beware of the preludes to zina, such as complacency with khalwa (private seclusion with a non-mahram) and immodest display.