Do we know it isn't? I mean, you have a very good point, and I didn't think of that problem - it is really not a secret you could plausibly keep for a thousand years, even with flawless commitment to family loyalty, because sooner or later someone will try to obliviate one of you and get away both alive and with their memory intact.

But on the other hand, Parsel-tongue seems like the kind of thing that's going to just accumulate rumors about it's properties like it was coated in velcro. Picking out which of the properties attributed to that ability are actually real from all the baseless rumors might be quite difficult. Especially if there are other defenses against obliviate.

.. And if defenses are possible, they have to exist. About 3 seconds after someone told me about memory charms and obliviate, I'd personally make "Become Safe From That Shit" priority number one, They're skincrawlingly, mindwarpingly awful spells. The kind of thing where if there isn't any other counters, I'd move to mars to preclude the possibility.