Is it illegal to throw away mail that was not addressed to you?

Your stepmother was not allowed to legally throw it out. What she should have done was locate you since you still live at the home. If you press charges, you would need to check with the postal inspector general to see if this is something they would prosecute and how far up the priority list it would be. Further, be prepared to show damages. If this was a bill, you have to mitigate your damages. This step could be contacting the creditor and explaining what occurred and asking for an extension so there are no damages to simply re-mailing what you intended to mail in the first place if it can be re-created.