Here's another example of why it's not a good idea to allow novice guests to saddle up, even if they are family members:
Canadian Rescue Sued by Relatives Over Horseback Riding Accident
Note that in this story, the hosts run a horse rescue and are fairly close relatives to the plaintiffs, but even that didn't save them from being sued! The good news here is that it appears the hosts had liability insurance. Although the hosts are quick to point out that a liability release might not have protected them, it's clear that there were no liability releases in place. Had the guests signed a properly worded liability release, it might have indeed protected the hosts, as it appears that the accident involved typically unpredictable horse behavior rather than some type of obvious negligence on the part of the hosts. Even if the liability release were not airtight, it would have provided the hosts' insurance company with a basis for negotiating a settlement with the plaintiffs.