What black-swan event would cause would 2 PiHoles go down simultaneously? You could always use a non-PiHole guest-network if your WiFi hardware supports it, and let your family know to use the guest network if the regular network is down. The manual switching might not be necessary as most computers, phones and tablets automatically disassociate from a WiFi network if it's "offline", such as when DNS resolution fails.
If you really want to try the pihole, and are worried about corruption you could use a read-only root filesystem and mount /tmp and /var/log on tmpfs ramdisk, alternatively, add "@reboot touch /forcefsck" to force a disk check whenever the Pi(s) reboot, including when power returns after an outage.
