It can be a bit confusing, can't it?

Sir Walter Elliot is the vain baronet we all want to emulate (kidding!) and it's Mr William Elliot who's his heir - chapter 1 describes his kinship to Sir Walter thus:

Heir presumptive, William Walter Elliot, Esq., great grandson of the second Sir Walter.

I don't think we clearly know at this stage how close the kinship of the two men is - cousins, &c. Or, for that matter, whose grandson, or great grandson, Sir Walter is. I guess if he's the baronet, he could be descended from the first Sir Walter. I'm sure someone will kindly set me straight if I've got it wrong, and I hope I haven't confused you further. :-)