The cousins have standing to challenge the validity of the amendment removing them as beneficiaries, but there would have to be some ground for invalidating that instrument, such as that your grandmother lacked capacity at the time she executed it or was unduly influenced to amend the trust. I'm assuming the letter from the attorney to which you refer was a notice under Probate Code section 16061.7. If that's the case, you and/or your cousins only have a very short amount of time within which to contest the amendment (60 days from the date the trustee provided you with the terms of the trust or 120 days from the date the notice was served, whichever is later). You should consult a trust litigation attorney to discuss the facts and consider your options. Depending on the value of the trust estate and the percentages that you were set to receive under the original trust, it may simply not be worth it for you from a financial standpoint to pursue a contest.
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Attorney Kaster's answer is right on point. Your question is simply put, can the trust be challenged; the answer is yes. However, just because the answer is "yes," it does not mean that such a challenge would be successful or worthwhile. As mentioned, you must consider whether you or your cousins believe (and may be able to prove) that either your grandmother lacked the capacity to make changes to her trust or she was the subject of undue influence which caused her to modify her trust. In either situation, you must also weigh the value of the 10% share of the estate versus the cost of challenging it.
Should you decide to proceed, the clock is ticking, you should contact an attorney immediately to consult on the matter.
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