Need help please.

My mom passed away in April of last year. She had two children-me and my sister. My sister had been estranged from my mom for years before my moms passing. My mother had a life insurance policy thru her job and in 2007 she named me as sole beneficiary on the policy (changing it from joint beneficiaries - me and my sister). She also changed her will leaving me everything and made me executor of her estate. When my mom passed my sister called the insurance company and stated that she wanted to contest me as sole beneficiary. Since then I have received papers from a lawyer representing the insurance company that I signed, had notarized and sent back as requested. Well, a few days ago I got a call from the lawyer who represents the insurance saying that my sister can no longer be located by them. The address she gave them is no longer valid and the phone number she gave them has been disconnected. The insurance company tells me they dont know what to do in this situation and the lamyer for them refuses to answer any of my questions. I dont know what to do to get the money that is owed to me. I really need it too because I have funeral expenses I have been unable to pay, real estate taxes on the house she left me that I am unable to pay, etc. The life insurabce company is Minnesota Life. Please, if anyone c an tell me what I should do to get this matter resolved I would be SO grateful. I am at a loss.