It's not uncommon for the descendants of slave owners not to know or remember that their ancestors owned slaves or for people whose ancestors were White supremacists not to know or remember that either.
If your ancestors were White and alive a century ago, it's not unlikely they were White supremacists, and if your family "owned a plantation" it was also very likely they owned people as well, but that's not something families talk about, especially if they've moved away.
I'd say it's human nature to forget such things or to conceal them to the point where one's children and grandchildren have no knowledge of the facts.
That may be true for some, but McCain celebrated his family history, with multi-generational naval heroes, and those ancestors fighting in the Civil War for the Confederacy. There were published accounts that discussed all these topics.
In an era when those praising McCain now, are the first to rip down Confederate statues, I find it ironic that they now keep a complicit silence.