Look so we can just get this out of the way, the main thing is the wiki, both of them, just state that Impa is a name of several female members of Sheikah. Doesn't actually say they are all descended from each other. Like Impaz is heavily implied to be descended from Impa but it couldn't be the same Impa from OoT because his Impa opened up the Hidden Village to outsiders aka had control over the village to do so. It does not have to be the same family line but just one of the Sheikah of the generation that happens to be named Impa. 

This about to overtake a whole page. lolz