Is the questioner sinful because he cannot reconcile with his father, and bears the expenses of his mother and younger brother, and supports his mother in her case against his father in court, while some people criticize him for that?

What your father is doing by abandoning your mother and young brother without financial support is a grave sin, as the Prophet (peace and blessings be upon him) said: "It is enough sin for a person to neglect those whom he is responsible to support." You must mend your relationship with your father and advise him to fulfill his duty of providing for your mother and brother. If he does not respond, there is no harm in filing a lawsuit against him in court to compel him to provide support, and assist your mother in that.