The password only keeps them from opening the folder without the password. If they have access to the account, then they have the same abilities as you to install or delete files or folders.
If you don't want someone messing with your data, then they should have their own account on the computer and log out of yours when your done. You can create an account each of the other family members, or create one account for the rest of the family to use and one for you. That way you can restrict what they can and can't do on the computer such as installing applications, etc.