The basic principle in Muslim transactions and contracts is validity
5-9-2023 | IslamWeb
Question:
I had technical problems, so if I ask this question multiple times, please don't be confused. ( I don't get the verification link ) السلام عليكم ورحمة الله وبركاته My mother wanted to divorce my father but chose khul. Her family tried to give back the dowry to him, but my father refused to take it and insulted her family instead. Although my mother has since remarried, I am unsure if the divorce is valid. As for my step-father, should I consider him a non-Mahram and wear hijab in his presence? My mother claims she asked scholars in our home country, who said it was the right thing to do. However, I find her explanation confusing, especially because there were no divorce papers, and she didn't have a Nikkah contract with my real dad either, as that is not how things are traditionally done in our culture.
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
As long as you do not know an apparent reason for the invalidity of your mother’s divorce or separation, as well as the invalidity of her second marriage, you do not have the right to declare it invalid merely because of doubt. The basic principle in Muslim transactions and contracts is validity. Al-Zarkashi
said in Al-Manthoor fi Al-Qawa’id Al-Fiqhiyyah: “The contracts concluded between Muslims are presumed to be apparently valid until their contrary becomes clear.” [End of Quote].
Accordingly, unless you know for sure the invalidity of your mother's marriage, her marriage is valid and her husband is a mahram to you and you do not have to wear Hijab in his presence.
You should know that if the marriage or divorce contract fulfills its conditions and pillars, it is valid even if it is not written or documented in the courts, as registering and documenting contracts is done for the purpose of preserving the rights and taking precautions and is not a pillar or condition for their validity.
Allah knows best.