Power of Attorney fraud.. plz HELP!?

My bf's dad appointed my bf's half-sister POA of his bank acct b4 his death. After his death, she changed his address, stole all his money and will not cooperate with my bf (the father's ONLY biological child) by supplying proof on what his father's money was used for. We've since discussed the matter with the bank, which stated that her POA ceased when the father died. We have a bank statement that shows withdrawels and forged checks from his bank acct after his death. We are not informed enough on what can be done to get this issue resolved. The main issue at hand is that he had thousands of dollars in checks coming in and my bf was left beneficiary but never saw a penny, in fact he was a minor child when his dad died and abandoned in the home his father left him, against the dad's wishes. The attorney who prepared the trust is basically ignoring my bf, probably bc he's so young and there are two half-siblings fighting to take his home away.. PLZ HELP!!