You present a very complicated set of facts and you do a pretty good job of it. But, this problem is going to require some analysis. It is possible to set aside agreements if one party was under duress at the time of signing or if a party was tricked or defrauded. However, your grandpa is now deceased and he was the one tricked not you. The fact that there was no probate on your grandma's Will and that it was intentionally kept from you may provide an opportunity for you. You really need to consult with an attorney who is not only experienced in probate law but in probate lawsuits. That attorney will need to develop all of the facts and a very specific timeline of events and that attorney will want to see the Wills of your grandma and grandpa and the "family agreement".
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
In addition to Attorney Olsen's answer, which I completely agree with, I would simply add that based on the facts you have presented, even if you *MAY* have had an action, at one point in time, it is very likely that the statute of limitations would bar any claim that you may have had. It is, after all, about 18 years since the date when your grandfather died. I do not believe there is anything you will be able to do anything at this time.
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
The facts need to be developed more for the proper analysis. However, it is possible that you may be able to have the agreement void if you were defrauded or were under duress etc. The case is complicated and you need an excellent attorney.