What should I do about the dispute that arose between my married sister and me because she demanded a sum of money, and my mother intervened against me, which led to my estrangement from both of them for four months?

You are advised to be keen on mending your relationship with your mother and sister, and not to enable Satan to sever kinship ties and cause undutifulness. Show kindness to them through financial support and service as much as possible, for honoring your mother and pleasing her are among the most emphasized duties. Allah the Almighty said: "And We have enjoined upon man, to his parents, good treatment. His mother carried him with difficulty and gave birth to him with difficulty, and his gestation and weaning are thirty months." (Al-Ahqaf: 15). And the Prophet, peace and blessings be upon him, said: "Who among people is most deserving of my good companionship?" He said: "Your mother, then your mother, then your mother, then your father." Narrated by Muslim.