Generally, a 16 year old could be named their own payee if they prove they are emancipated and living on their own. Or are 17 1/2.
Hopefully you have a separate bank account for the direct deposit of the survivor benefits and can show that you have saved it all and that it matches with your annual payee accountings. You can show this to the SSA office. I am assuming there is no court order for custody since you are a natural parent, but if you have any documents (school records sent to your house) showing that the 15 year old still lives with you and not the grandmother. You should file to be named payee again, and possibly talk to an office supervisor. A 15 year old should not be named as their own payee. But at this point, it would be helpful for you to supply some documents that support what you say if the grandmother is not telling the truth.
Hopefully you have a separate bank account for the direct deposit of the survivor benefits and can show that you have saved it all and that it matches with your annual payee accountings. You can show this to the SSA office. I am assuming there is no court order for custody since you are a natural parent, but if you have any documents (school records sent to your house) showing that the 15 year old still lives with you and not the grandmother. You should file to be named payee again, and possibly talk to an office supervisor. A 15 year old should not be named as their own payee. But at this point, it would be helpful for you to supply some documents that support what you say if the grandmother is not telling the truth.