What is the ruling on desiring to marry a Shia girl, with the family rejecting this due to sectarian differences, while the relationship continues and the brother is influenced by Shia ideas and refrains from fasting Ashura?

The Prophet (peace be upon him) informed us that the Muslim nation would split into seventy-two sects, all of which would be in Hellfire except one, which is Al-Jama'ah (the community). The Prophet (peace be upon him) clarified the saved sect by saying: "That which I and my companions are upon." This description applies to the People of the Sunnah and the Community (Ahl al-Sunnah wal-Jama'ah), who adhered to the path of the Companions in belief, action, and conduct. As for the Shi'ah, their شعار (motto/sign) is hatred and declaring the Companions as disbelievers, extremism regarding the Household of the Prophet (Ahl al-Bayt), believing in the distortion of the Qur'an, and their imams claiming knowledge of the unseen. Slandering the Companions is slandering Allah and His Messenger (peace be upon him). Therefore, one must cut ties with those who invite you to their false doctrine and beware of their misconceptions, and not be deceived by the adornment of their speech which is based on taqiyyah (dissimulation). It is also advised to avoid reading the books of the people of innovation and misguidance, except for those who are well-versed, and to present anything that causes doubt to the people of knowledge, because doubts are captivating. Fasting the day of Ashura is recommended and established by authentic hadiths and the consensus of scholars. One should frequently supplicate to Allah Almighty and ask Him for steadfastness.