Must a betrothed woman inform her fiancé about her previous unregistered marriage (zawaj urfi) that was not consummated, and would she be sinful if she did not inform him for fear of spoiling the engagement?

If the 'Urfi marriage (customary marriage) fulfilled the conditions (guardian and two witnesses) but was not registered or publicly announced, then it is valid. The woman is not required to inform the suitor about it unless conjugal relations occurred or the suitor inquired about her marriage.

However, if the 'Urfi marriage did not fulfill the conditions (e.g., without a guardian or witnesses), then it is forbidden and invalid. In this case, she must repent and conceal her secret. She is not required to inform the suitor about it, even if he asks her, based on the Prophet's (peace be upon him) saying: "Avoid this filth which Allah Almighty has forbidden. So, whoever falls into it, let him conceal himself with the concealment of Allah Almighty." And "Allah does not conceal a servant in this world except that Allah conceals him on the Day of Resurrection."