If you need to use more than one account (e.g., for a sibling or a secondary "smurf" account), Steam allows this officially:
Yes, you can access different Steam accounts from one computer. However, you may only access one account at a time. Account Security Recommendations - Steam Support Download 16x Steam Accounts txt
The safest way to manage multiple Steam accounts is through Steam's built-in features and official security tools. Why You Should Avoid Downloadable Account Lists If you need to use more than one account (e