I have a son now 11. Sons father left as soon as the line turned blue never to be seen again. I received through attachment of earning from CMS maintenance. Today I received a letter saying the amount I'm getting will be reduced as his father is now living with someone who receives child benefit. So does that mean my son has a brother/sister or is this just a live in girlfriend child? Would the CMS tell me if I asked? Would I have to go to the registers of birth and ask? They stay less than a mile away and I don't know what to tell me son if truth be told. I'm not overly interested but I would appreciate knowing if he has another biological child.