Remarrying a woman according to Shari`ah after terminating a Shighar marriage with her

Q: My father made me marry a sister of a person who in return married my sister (with no mandatory gift to brides from grooms). It was all done out of lack of knowledge. Later on, we knew that it was unlawful, so I divorced my wife and so did my brother-in-law. After that my sister married another person. Now my ex-wife refuses marriage to anyone other than me. It should be noted that I had two children from her. Is it permissible for me to remarry my ex-wife? Should I give her a new Mahr (mandatory gift to a bride from her groom) and conclude a new marriage contract after about three years of our divorce? I would like your Eminence to tell me what I should do.

A: It is permissible for you to remarry your ex-wife with a new Mahr and a new marriage contract just like any other woman. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.