I am beneficiary to my ex husbands 401k. His POA who was his stepsister rolled the money over to another account. Can I get it back.

My ex husband has me as beneficiary for his pension because we still have to minor children together. He also has a adult son who I found out also was his POA. While family was visiting my EX in the hospital he was ask if he wanted to change anything including his benificiary for his pension. He replied NO he didn't want to change anything and he wasn't going to sign anything. On the day of his death the pension was rolled over to another account. His stepsister and son made themselves benificiary to the new account. I believe he wanted me to be benificiary because I would have been fair to all three of his children. Do I have any rights here? Or should I forget about the money and move on?