All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The Sharee'ah evidences indicate the prohibition of a woman traveling without a Mahram. We mentioned some of this in Fatwa 83795, so please refer to it. These evidences are general for both the young lady and old woman. Some of the legal scholars, like the Maalikis, make an exception for the old woman who reached an age in which men no longer desire her, and there is no specific age for that.
If your sister is in a condition in which traveling without a Mahram is not allowed, then you must advise her against proceeding. If she insists, then you are not permitted to assist her in that. Shaykh Ibn ‘Uthaymeen said in the course of answering about sending for foreign maids without a Mahram: "It is not a valid excuse to say that the sin of her traveling without a Mahram is only upon her or upon the office which sent for her. This is because making way for someone to commit something forbidden is to be a partner in that sin by way of assistance, as Allaah The Exalted said (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression.} [Quran 5:2]."
Allaah Knows best.