You need to copy the password file from the primary to the standby and then rename it for the standby. Then cancel apply and restart the database using the new password file.
The standby password file must be an exact copy of the primary password file. In fact all nodes in Data Guard must use the same copy of the password file.
So when you make a change on the primary nothing happens to the password file on the standby.
So just changing a password on the Standby is a bad idea. You will probably break Data Guard. Make your changes on the Primary and move the password file.
Mseberg explained this problem clear and completely. I want to approach differently. If password changing of sys user is inevitable fro you Primary database each time , If you database 11g you can use REDO TRANSPORT USER
see following link: Mahir M. Quluzade | Oracle Blog: Oracle Data Guard 11g - Redo Transport User