Can somebody sign into my account even if I have it as primary
Yes, you should change the password immediately.
Non-Primary accounts are able to spend funds in your wallet. Also be aware that there is a limit of sign-ins per account, to play games or watch videos.
https://www.playstation.com/en-nz/get-help/help-library/my-account/device-activation-deactivation/about-device-deactivation/