What does Islam say about a sister who damaged the relationship between the son who is 36 years old and his entire family by fabricating a story about the lady he is marrying and hence the son's mother disowned him?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Having good relations with relatives in general and with blood relatives in particular is a great obligation on every Muslim. This fact is established in many texts of Shariah. No doubt, whoever tries to sever kin relations through talebearing even though they are true commits a great sin since he/she combines two great sins, i.e. talebearing and severing kinship. If accusing people and fabricating stories against them is added to the above two sins, then the matter becomes greater and the sin becomes very awful and repulsive. Therefore, every Muslim should fear Allah regarding others. Allah Says (interpretation of meaning): {Would you then, if you were given the authority, do mischief in the land, and sever your ties of kinship?}[47:22]. He also Says: {And those who annoy believing men and women undeservedly, bear on themselves the crime of slander and plain sin.}[33:58]. The Prophet said: "A talebearer does not enter in the Paradise". [Reported by Imam Muslim].
No doubt what your sister did is evil. Despite that we advise you to treat her in good manners and compensate her for her bad deeds with forgiveness, showing affection to her. Allah Says (interpretation of meaning): {…Repel (the evil) with one which is better (i.e. Allâh ordered the faithful believers to be patient at the time of anger, and to excuse those who treat them badly), then verily! he, between whom and you there was enmity, (will become) as though he was a close friend. But none is granted it (the above quality) except those who are patient, and none is granted it except the owner of the great portion (of the happiness in the Hereafter i.e. Paradise and in this world of a high moral character)} [41:34-35]. It is narrated in a Hadith that a person came to the Prophet and said: "I have relatives with whom I try to keep the ties of relationship but they sever relations with me; and whom I treat kindly but they treat me badly, I am gentle with them but they are rough to me.'' He (Blessing and peace be upon him) replied, "If you are as you say, it is as if you are feeding them hot ashes, and you will be with a supporter against them from Allah as long as you continue to do so"[Muslim].
Allah knows best.