Change the current administrator password, so you can login. Once you're logged in, make a new administrator account for you, and using that, make the current one a non-administrator. And make an account for the other sibling also.
https://www.howtogeek.com/222262/how-to-reset-your-forgotten-password-in-windows-10/ tells how, with the utilman trick.
A new computer now uses Windows 10. Vista was last sold in 2010. If you don't have a Windows 10 install disc, make it from https://www.microsoft.com/en-us/software-download/windows10
In a home with two siblings, one has designated himself as the computer administrator so that the other cannot use it. How the heck do I override the vengeful child's administrator password, and make one parent the administrator instead without having to take the tower to a service provider and have to pay for them to do it? Can I also prevent anyone else from changing the administrator password once I make this change? This is a fairly new computer, bought in the last year, so I suppose it's Vista 64 bit, but am not sure. it's a windows OS.
Thank you very much.

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