I would recommend running untrusted proprietary software as a webapp if possible, it will not be able to be as intrusive in that case compared to native app even with all the sandboxing (flatpak, snap, etc…). Or Virtual machine as the most secure option, but it may be overkill.

Unfortunately you have to create an account and provide phone number to WhatsApp for communication with your family.

You can disable cloud backups, set auto destruct message timers, not use metaAI and not put any extra info, but not much else to mitigate. At least message or voice content is end to end encrypted, but Meta sure collects a lot of metadata.

Also if you are on grapheneos, you can set up contact scopes, to only include people you communicate with on the service.