Will my inability to forgive my sister before Hajj, due to her mistreatment of my mother and us, affect the validity and acceptance of my Hajj?

Severing ties of kinship is a major prohibited sin. Maintaining kinship ties is required even with those who cut them off, based on the Prophet’s (peace be upon him) saying: "The one who maintains ties is not the one who reciprocates, but rather the one who, when his relatives cut him off, maintains ties with them." The wrongdoing and injustice of relatives do not negate their right to be treated kindly. Therefore, do not cut off your sister; maintain ties with her kindly and advise her. If you have fulfilled your duty and she insists on severing ties, then the sin is upon her. As for Hajj, its validity does not depend on maintaining ties with your sister, but it is recommended for the pilgrim to repent and redress grievances.