What is the Islamic ruling regarding what should be done concerning the first fiancée and the proposed second wife, given the inability to be frank with the current fiancée and her family?

You have committed a grave sin by corrupting a wife against her husband, as the Prophet (peace be upon him) said: "He is not of us who incites a woman against her husband." However, your sincere repentance is likely to be accepted, for Allah the Almighty says: "And indeed, I am the All-Forgiving to whoever repents, believes, does righteousness, and then is rightly guided" and "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'"

It is permissible to marry the woman after her divorce from her husband, whether you marry her alongside your first fiancée or not. If you marry them both, the fiancée's consent is not a condition, but you must be just between your wives, as Allah the Almighty says: "Then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one."

Regarding the secrecy of the marriage: - If what is meant by it is the absence of witnesses, then the marriage is invalid. - If what is meant by it is concealing it from people while all the conditions of marriage are met, then the preponderant opinion is that it is valid.