Well, from your description it certainly sounds like something improper could have gone on here. You need to retain an estate litigation attorney immediately. The only grounds to have this amendment excluded would be undue influence/coercion or lack of capacity. Proving any of these is difficult and rarely happens. You will most likely need medical records and expert testimony. This will probably be a long protracted battle so be prepared for that.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
If it can be proven that the trust amendment was executed while your grandfather lacked capacity, then the amendment could be invalidated. Essentially, then, the terms of the trust would return to where they were before the amendment was signed.
As well, your aunt may be liable for financial elder abuse, which includes many kinds of financial wrongdoing.
I recommend speaking with an experienced trust and probate attorney regarding the possibility of contesting the trust amendment and other courses of action.
DISCLAIMER: THE INFORMATION PRESENTED HERE IS GENERAL IN NATURE. IT IS NOT INTENDED, NOR SHOULD IT BE CONSTRUED, AS LEGAL ADVICE. THIS RESPONSE DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP BETWEEN US. YOU SHOULD CONSULT WITH A QUALIFIED ATTORNEY FOR SPECIFIC LEGAL ADVICE ABOUT YOUR PARTICULAR SITUATION.
Someone will need to contest the validity of the amendment. Undue influence or incapacity are likely to be the basis for such a challenge. You will need a good estate litigation attorney and should begin seeking representation. Beware, there may be a no contest clause and if so that could result in disinheritance if you were to lose.