Should the couple remarry, or is it better to inform their families of their marriage which took place three years ago, taking into consideration the problems that would result from that, after the families initially refused their marriage on the pretext of family customs that dictate the marriage of the eldest daughter before the younger?

Marriage is not valid without a guardian (wali). If a guardian prevents his ward from marrying a suitable man whom she has accepted, he is considered to be unjustly obstructing her (aadil), and his guardianship falls away and transfers to the next in line. His preventing the marriage of the younger daughter before the older is against Islamic law, and guardianship transfers to the more distant guardian or to the legitimate ruler (hakim shar'i). If the one who married them off was a legitimate Islamic judge, then the marriage is valid. But if he was a non-Muslim judge, the marriage is not valid, and the contract must be renewed with a guardian.