Is the questioner considered to be severing ties of kinship for not re-establishing his relationship with his sister, who caused problems between him and his son, knowing that he contacts her on special occasions and fears that she will stir up problems again?

Forgiveness and pardon are characteristics of good people. Allah Almighty says: "And the recompense of an evil deed is an evil deed like it; but whoever pardons and makes reconciliation - his reward is due from Allah. Indeed, He does not like wrongdoers." Take the initiative to reconcile and forgive, and advise your sister gently and kindly regarding what she has done, for religion is advice. What she did is فساد ذات البين (causing discord between people) and inciting disobedience to parents, and it is obligatory for her to repent from that.