With such a brief synopsis as this forum allows, the best answer is maybe. Your biggest hurdle is the fact that you have waited so long to ask this question. There are time limits for filing various claims, so my recommendation here is to get to an estates and trust attorney immediately to determine if you (1) have missed the deadline, (2) have a claim.
This answer is for informational purposes only and should not be considered specific legal advice, nor does it constitute an attorney-client relationship.
That is about the saddest thing I've heard today, and one sad part is that everyone involved was probably trying to do the "right thing."
Since both your parents are now deceased. you would have a very hard time overturning the trust in favor of your youngest sister. If you're going to try, don't let any more time go by, get to a Missouri attorney who does estate litigation. He or she can explain better than I the requirements of Missouri law. You are going to need proof that either your father was of unsound mind when he signed that trust - whether due to his medical condition or his medications or whatever, or that your sister exerted an undue and improper influence over him, in effect taking away his free choice in the matter. This could have been done by any number of means, but merely showing that she was kind to him, and assisted him is not going to be enough. It is very much an uphill struggle.
Is there any chance you can sit down and reason with her? Discuss your other sister's needs, and your own concerns. Try to make her understand the essential unfairness of this outcome.
First seek the advice of an estate litigator, then decide which course of action to follow.
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In Florida we have a presumption that if an heir takes the decedent to a lawyer selected by the heir to do a will, that there was undue influence on the decedent, which makes that will void. You should consult a lawyer to see if there is a similar law in Montana.
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