I would recommend consulting with an attorney. I would also recommend that you contact the Social Security Administration, if not already done, to file a claim for survivors benefits since she is a minor. You may be able to determine his SSN from her making a claim for survivors/dependents benefits. It is not entirely clear to me what this situation is with the "insurance claim" but if she was listed as a beneficiary, then it should pass (or some portion should pass) to her assuming that the payments continue after his death. I would recommend keeping in touch with the attorney that you've identified as being involved with that matter. I wish you the best of luck with this situation.
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Yes you probably need to hire an attorney. At the very least, you need to make an appointment with an attorney to discuss these matters in detail. If the only issue dealt with her entitlement to SS benefits, than you might be able to handle it yourself through conversations with the Social Security Administration. However your nieces situation is more involved. You apparently do not have some of the information you need in order to deal with SS. Further there are also others who are already trying to get insurance benefits to which your niece is entitled. As her legal guardian, you have a legal obligation to protect her interest.