If you believe that your ex-husband's family has cheated you out of benefits which should have come to you and your son--for example, that the sister or her family took all of the life insurance money, instead of sharing it with you--you could sue her for theft: for stealing money to which you are entitled. In the course of that lawsuit, you could use legal process to get a copy of any policy, any correspondence with the insurer, any checks or other payment, etc. However, you can only use that legal process to compel the release or production of information or documents in a lawsuit, not otherwise, which means you'd have to sue before knowing 100% that there is anything to sue over. Other than simply contacting every insurer out there and seeing if any of them do have a record of the policy, suing is the only way forward for you, and, as stated, could result in you filing a pointless lawsuit.