What is the religious ruling on marrying a man of unknown lineage, knowing that he is of good religious commitment and character, and of high social standing, and that the family's refusal is based solely on their fear of relatives' reactions?

The basic principle is that it is permissible to marry someone whose religiosity and character you are pleased with, based on the Prophet Muhammad's (peace be upon him) saying: "If someone whose religiosity and character you are pleased with proposes to you, then marry him [to your female guardian]. If you do not do so, there will be tribulation on earth and widespread corruption."

Therefore, try to convince your mother, with the help of benevolent people, that the suitor being of unknown lineage does not harm him, because "Indeed, the most noble of you in the sight of Allah is the most righteous of you."

If she agrees, then all praise is due to Allah. Otherwise, obey your mother, because obeying her is an obligation upon you, while marrying him is not.