This looks like a setting that's supposed to allow for, or establish, the spline of the cover. Instead of dabbling with the front and back cover settings, look at your spline borders and bleed settings.

I've thought about adding a family tree, but it would be HUGE.  Three families, four timelines, lots of extra characters that marry into the families and are important to the series.  Now that we're adding more books, it's probably a good idea for new readers.

I like the idea of having a PDF on the website.  That would be much easier.

I also have three families over three generations, which does make it tricky for readers if they put down the book for a few days and lose track of who is who. How would you link to your website? Would you put a note in the beginning of the book explaining about the family tree and leaving the link?  What about the print version? If they have to check the link every time they open the book, or lose track, wouldn't it eventually become a bit onerous?

As far as size, for paperbacks or hardcovers, you can cut it into two pages, Left and Right, so the whole thing will fit. Ebooks don't really matter because kindle readers, phones, apps, etc, can focus in on the image.

When I did my family tree for my Azure series, I focused more on appearance than content. In other words, I was looking more for it looking pretty than its actual content. If you click on my newest cover under here in my signature, Princess Sojourn, it'll take you to Amazon where you can look at page four in Amazon's sample. There's my family tree for the royal family in my epic fantasy series.

This is supplement content so I would focus more on appearance than content. I think the appearance is super important, if you're gonna do it.

Thanks. I checked out your fancy family tree. Looks good. 

My family tree is important because of the large cast of characters. I always intended to put one in the front of the book but couldn't work out how to do it so published the original without one. But a reviewer said they had a hard time keeping track, so I persevered and finally figured out how to do it. 
As you mentioned, the ebook readers could enlarge it, so I won't worry about that.
Thanks for the suggestion about doing it over two pages. If I were to add another page it would mean reformatting all the chapters so they still open in the correct manner. I can see myself messing it up  As very few print versions get sold I'm probably better off leaving it as it is. I'm going to get a new cover, so might ask the cover designer to do it as they include a title page in the quote and might be open to an extra page. 

Flow Chart Software could produce that layout.
 
I use XMind (free) these days, but I think even Microsoft has a version of flowchart software. I also think you could create that layout in a Spreadsheet.

Thanks. I was hoping to use something I already know my way around (although I can't even remember how I did it in the first place, but I do know it took a lot of time and effort. ). It would probably take me less time to draw it by hand, scan it in and save as a JPeg.

With a family tree that size, that may be the best that you can do.

When I was creating genealogies for my Greek mythology book, I tried to do it in Word, which is complicated because they never came out right when converted to ebook. I explored genealogy software. That was a few years ago (2017), so the landscape may have changed, but most of them were geared for small, simple family trees. I ended up using SmartDraw (which is more expensive now than it was then), only to discover that saving as an image kept creating something that expanded file size too much to be practical in an ebook. Probably, I could have found a way to convert it to something that worked better, but I was working under time constraints. I ended up including the genealogies in the paperback only. For the ebook, I linked to my website, where readers could download the genealogies as a PDF file.

Thanks, Bill.
Your advice will save me a lot of angst and I'll leave well alone. I've tried 'fixing' things before and ended up worse off.