What should the family do if a man proposes to their sister who has lost her virginity and had an abortion? Should they tell him the truth or conceal the matter, and what are the potential consequences of each option?

If the sister repents and her repentance is sincere, you are not obligated to inform her suitor of her past action. This is because the guardian is not required to disclose a flaw or a past transgression of his ward if she has repented, as indicated by the statements of scholars and the action of Umar ibn al-Khattab, may Allah be pleased with him, when he commanded a man to marry off his daughter, who had committed fornication, after her repentance, without exposing her.