I checked a bunch of places for this last month because my library holds were taking forever. The absolute most reliable way is through your local library's digital service, like Libby or Hoopla. You just need a library card, which is free, and you can borrow it just like a physical book. Availability depends on your library's specific collection, but I've seen 'The Hawthorne Legacy' pop up in several systems. The waitlists can be long for popular titles, though. Sometimes putting a hold on the first book 'The Inheritance Games' can get you faster access to the sequel, since some people start the series from the beginning.
If your library doesn't have it, there are a few other legal spots. Spotify Premium now includes 15 hours of audiobook listening per month in its subscription, and I'm pretty sure I saw the series in their catalog. It's not fully "free" since you pay for Premium, but if you already have it for music, it's a nice bonus. Also, services like Audible often give you one free credit when you start a trial—you could use that to get the audiobook and then cancel if you want. Just make sure you follow the cancellation steps so you don't get charged. Scribd's subscription model might also have it, where you pay a flat fee for access to their entire library, audiobooks included.
I'd steer clear of random YouTube videos or sites claiming free full audiobooks; those are almost always uploaded illegally and get taken down fast. It's frustrating to get halfway through and lose your place. The library apps are the most straightforward route, even with the wait. My copy just came through last week, and the narration is really engaging—the voices for the Hawthorne brothers are distinct enough that you can tell them apart easily.