Hi David,
Yes, please go to https://account.microsoft.com/devices and see if you can find the computer in ‘Find My Device’ tab. But if it's not connected to the internet you may not be able to find it. And then select 'Remove device' so that others cannot use it to log in to your sister's Microsoft Account anymore.
The final suggestion is to reset her Microsoft Account password, and enable MFA (mobile or authenticator), so that can make the account more secure.
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