Is it permissible for a betrothed woman to speak with her fiancé during Ramadan without her family's knowledge, and is her make-up fast valid in this case?

Your fiancé is a stranger to you, and excessive conversation with him is a gateway to evil and corruption, and a following of the devil's footsteps. Therefore, you must repent and abandon the impermissible conversations between you until the marriage is concluded. Be aware that such conversation does not invalidate the fast unless it leads to the emission of semen.