Is one who quarrels with his siblings sinful for their speaking ill of him in his presence or behind his back, constantly criticizing him, seeing only the bad in him, and forgetting themselves?

If estrangement means boycotting, then it is not permissible for a Muslim to boycott his brother for more than three nights. The better of the two estranged persons is the one who initiates the greeting. Boycotting among relatives is a greater sin. Boycotting is permitted for a legitimate religious benefit, such as boycotting for the sake of sins and innovation, provided that it leads to repentance, or if one fears that associating with the other will corrupt one's religion or harm one's worldly life.