Shanks has two options backstory wise.
He's either related to Xebec, given he was found at the Rox crew's defeat.
Or he's a Tenryuubito.
I think the first is a little skeptical because the Rox legacy pirates, Kaido + Linlin, haven't hinted at this in the slightest. So odds are he is descended from the Tenryuubito and whoever this guy is, is a close relative.