Hi, My name is Philip. I am an attorney with over 16 years experience. Hopefully I can help you with your legal question.
That is true (that is, your grandfather was accurate when he stated you can not claim you own support)/
Child support is designed, and is part of the law, to allow supplemental income to the parent raising the child. So in your case, the support (for you) would go to your mom. That is how the order (the court order) is written. It orders (in your case) the father pay the mother
There is no way to request that the support be paid to the child directly.
Now...if, for example, you were to take custody of one of the younger children (say your mom was not capable for some reason or another)? In such a case, as part of the court order granting you custody of the child, you could ask the court to order the father to pay you directly. This would be possible.
But not to yourself for the support designed for you
Sorry to have to bear bad news.
Let me know if you have more questions, happy to help if I can