Survivor benefits

So not to make this a long story and to get to the point. I am a SSDI recipient, my middle daughter who is adopted gets a family benefit along with my biological son. My adopted daughter’s biological father passed away a few years back after I adopted her. A few months back his father called my wife up and told her to get benefits under him. Well my wife recently submitted all the paperwork and they told her if my adopted daughter is eligible she will get 6 months backpay. Now because each one of my kids get benefits from me they split it. My question is , if my daughter entitled to her biological father survivor benefits and if so will they have to go back and pay my son 6 months my full benefit if they have to pay my daughter hers from her biological father. From what I read the answer is yes but that is my interpretation of the rules. Thanks for your help.