Are you the administrator on the Mac? Was your brother in law using a standard user Account on the Mac? If yeses, then you could delete his standard user account. And see Eric Root's link on deleting an Apple ID, but you also mentioned disabling one. In this context, you can deactivate one. Note that Apple prefers deactivation for the obvious reason mentioned below. So, if still are thinking of disabling/deactivating or deleting, see:
How to deactivate your Apple ID account https://support.apple.com/en-us/HT208503
If you request account deactivation, you'll receive a unique alphanumeric access code. When you're ready to reactivate your Apple ID, contact Apple Support, provide your access code, and we’ll help you reactivate your account. If you lose this code, we won't be able to help reactivate your account."
How to Delete your Apple ID Account https://support.apple.com/en-us/HT208504
“While your request is being processed, you can contact Apple Support with the unique alphanumeric access code you received, and we’ll help you cancel your request. After your account is deleted, Apple can't cancel the request, reopen your account, or restore your data.”
You can get additional information on both by signing in here: Data and Privacy https://privacy.apple.com/account