Stepchildren are entitled to draw dependent benefits in certain circumstances. If your father was providing at least one-half of his stepchild's support at the time he became disabled, then the 17-year old is legitimately drawing dependent benefits. Also, he must be a full-time high school student.
There are auxiliary benefits for stepchildren; the circumstances you described would be an appropriate scenario.
Benefits for children continue until the age of 18, with a maximum age of 19 if the child is still in high school. Because these benefits are based upon the parent's disability, there is no stringent obligation to spend the money on "the child's needs" per se whereas if the child was disabled, there are limited allowable expenses, and the representative payee must provide an accounting of all expenditures. Auxiliary benefits for children "make up" for what the disabled parent is unable to earn due to the disability.
Keep in mind that paying the mortgage, utilities, groceries, etc. are for the child's benefit.
If you're concerned that the child is getting "double benefits" because of the child support, that is a matter for friend of the court. Social Security auxiliary benefits are not impacted by that kind of child support in that way. Often auxiliary benefits are viewed as in lieu of child support when it is the disabled person that has to pay. That is not the case you described.
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