Is the repentance of a woman who committed a sin, then repented with sincere repentance, and vowed to herself not to return to sin, accepted, but she fears God's punishment by being deprived of childbearing due to her cousin's curse upon her?

Your repentance is accepted as long as you are sincere in it. Allah does not answer prayers that involve sin or severing ties of kinship. Your relative's prayer for you to remain childless will not be answered because it is a sin, an injustice, and an act of severing ties of kinship. Continue to be righteous and do not fear her prayer.