Is she sinful, and what is the expiation for plucking her eyebrows on her wedding day if she was forced to do so, with her intention not to repeat it?

Nams (plucking eyebrows) is among the major sins. Indeed, its doer has been cursed in the noble hadith narrated by Ibn Mas'ud (may Allah be pleased with him): "Allah has cursed the women who practice tattooing and those who have themselves tattooed, and the women who pluck their eyebrows and those who have theirs plucked, and the women who make spaces between their teeth, for beautification, changing the creation of Allah." Therefore, it is not permissible to do it, not even once, even if it is on the wedding day. It is obligatory to avoid disobeying Allah and incurring His wrath.