I am sorry for your loss, to get the Bitlocker Recovery Key, you would need to be able to access your uncle's Microsoft account on the link below, that is the only place the Bitlocker Recovery key would be stored, and no-one else can get that key for you, even if you contact Microsoft Support, they have no method available to access the Bitlocker Recovery Key.
https://account.microsoft.com/devices/recoverykey
If you are unable to access your uncle's account, then I am sorry, you will not be able to access the data on that drive, the encryption is too strong.