Why not just call and straighten it out?
You are correct the survivor's benefit continues until your 19th birthday if you are still in high school. You are incorrect that at the age of 18 these benefits are paid to you. They continue to be paid to your guardian since the premise is that you are not living on your own yet.
I'm not sure if you can sue just bc she is your guardian. I'm not sure how social security checks are, but if they're like a regular check then your name should be on the checks and the bank should only let you cash it. but if you have a credit card or bank account there may be a way so that your check gets deposited right into your account every month