Elderly impaired relative and co-trustee to an estate

In 2001 an oil company sent my elderly mother who had had two strokes and was living in an assisted living facility because she could not take care of herself.

Secondly, our ranch property is part of a trust which my sister is a trustee along with my mother, and she did not sign the agreement because she was never contacted about said amendment to the original lease.

My question is there just cause to set aside this agreement for both reasons listed above because a) the elderly relative had diminished mental faculty was asked to a sign a legal document that was in 6 point font and she did not understand and could not read because of both medication and cataracts and, 2) because it did not have my sister's signature on it as well.

If so, what is the statutory, common law or administrative law decisions of the Texas Railroad Commission to have said amendment set aside until we retain an attorney to take up the matter for us? Also, has the Texas Deceptive Trade Practices Act ever been used successfully to negate a oil and gas lease amendment in the State of Texas?