My father, passed away Sept. **, ****. His Facebook account is still active & is being...

My father, passed away Sept. **, ****. His Facebook account is still active & is being controlled by his common law wife, aka, my step-mother. The problem is that under normal circumstances, she would have no way to access his account. Meaning, she does not know the email he used, nor his account password. She has control, due to her possessing his personal phone, which is still logged into his account. I believe there to be incriminating evidence against her, in his FB messages. Namely, messages between him & my birth mother. I do not want the account deleted or memorialized, just yet, due to the severity of possible incriminating evidence against step-mother. My question is, what would I need to do to prove I am his only, biological child? What would need to be done to gain access to his account & messages? Would it require a court order? Is this something I would need to hire a lawyer to handle? I am at a loss of what to do, if anything can be done. Any help would be greatly appreciated. Thank you.