I am in process of getting married as a seconded wife to my fiancée; he does not want to tell his current wife about this marriage. How will he treat me equal if she doesn't know of the second marriage? Isn't he required by Islamic law to tell the first wife and to treat us equally in time and other rights?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. Basically a marriage contract should be performed publicly and should be made known to the people, in order to distinguish it from the illegal contracts. The Prophet (Sallallahu Alaihi wa Sallam) said: Make the marriage publicly known. [ Ahmad ] This is a likeable practice; otherwise, informing the first wife or the second wife or the children is not a compulsory condition of a valid marriage. If the marriage contract is established fulfilling the compulsory conditions such as determining exactly who the bride and groom are, having the consent of the guardian of the woman, doing the contract in the presence of two witnesses, etc., then this marriage is valid if no other legal thing that hinders legal marriage is there. Therefore, if the marriage contract is performed fulfilling all the conditions then it is a valid marriage even though the wife and children are not aware of this marriage. But it is compulsory on the husband to deal justly among his wives. To know more on this topic, please read the Fatwa: 84282 and 85398. Allah knows best.