Your only recourse would be to sue your sister and the insurer you would sue for breach of contract you believe you were a third-party beneficiary of the contract and for theft your sister stealing  from you by changing the paperwork. In the course of the lawsuit, you would be able to get additional information and paperwork. That material might not support what you believe--it might show that your mother did exclude you. Or the information may be ambiguous and you may never know for sure. However, only in a lawsuit will you have access to legal tools "discovery" written questions, document requests, depositions, etc. that you can use to get answers. So even though the lawsuit might well not result in you getting any compensation, it's the only way to try to find out what happened--and if there is evidence that your sister altered the paperwork, you might be able to then recover compensation.