Based on your description, it sounds as though opening an estate may be your only alternative. You should discuss the entire situation with a lawyer who regularly litigates in probate court. What you are planning on doing is not cheap and, in the end, may not be successful.
Best of luck.
I am licensed to practice law in Michigan and Virginia and regularly handle cases of this sort. My answering your question does not establish an attorney-client relationship. You should consult a lawyer so you can tell the lawyer the entire situation and get legal advice that is precisely tailored to your case.
If the advisor will not speak to you, then you can assume you are no longer a beneficiary. Otherwise, they would send you the claim forms to get your share.
If you want to try to undo what your sister has done, you will need to open a probate estate with the court. The will does not control the brokerage account, but there may be some arguments to be made that the 7 were intended beneficiaries and that your sister, through some wrongdoing, subverted your mother's intent. You will need to hire an attorney familiar with probate litigation.
Many of us offer a free phone consultation, so it's worth making a call to get more specific information!
I am very sorry for your loss and that you are dealing with difficult family dynamics on top of that.
I agree with my colleagues. You are in. Tough position. it may be possible to undo the beneficiary designation, but it will take the assistance of a very skilled probate litigator and a lot of facts that are compelling. It is not clear from your summary whether you have any reasonable likelihood of prevailing or not. That is where an attorney can help, at this point.
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the best thing you can do is contact a probate attorney and discuss all these is sues with him he will know which route to take