Depends on how much you trust your family with your rig 😂.
We generally lower our rates by $50 per day for friends/family if its summer, and sometimes a lot more in winter if our RV was open otherwise.
Outdoorsy allows you to create custom quotes for specific people so you don't have to mess with special price rules or anything like that. From your Dashboard, go to the "Bookings" tab, and there is a big "Send a Quote" button in the upper-left. You can set up the dates you want, adjust the price however you want and send it to your family member's email. Alternatively, if you have already been messaging your family member through Outdoorsy, there is a "Send pre-approved Quote" button on the messages screen you can use to accomplish the same thing.
To create a discount code, log in to your dashboard, then go to account/settings, you will see a tab for discount codes. When creating discount codes make sure they are not easy to guess. You do not want your renters “guessing” their way into a discount. The percentage is really up to you as the owner. You can also go the route of sending a custom quote like @JordanB explained.