I also think he meant people in the Uchiha that plotted the coup d'etat. It might be his parents, it might be others, but they will definitely be Uchiha. He cannot ask [yet] Hiruzen, he cannot further ask Danzo [dead], Konoha elders [where? finding them would cause further problems for Sasuke anyway].
Orochimaru could possibly explain, but he isn't the type. He would rather use some sick technique to show his prowess and have his technique do the rest [Edo'ing dead Uchiha].
And assuming seals can be unsealed, which we know can be done i.e. any Jinchuuriki seal, then yes technically Hiruzen, Minato, Hashirama, etc. could all be brought back [by undoing the Reaper God technique], but ultimately that would require living sacrifices to bind the soul to a body and that's not really the "good guys" way of doing things.
Perhaps, just as sealing techniques were considered godly [the Uzumaki were famous for their seals and we know how strong they can be if they can hold back not only attacks but the Bijuu itself], as may be techniques of unsealing [probably even more legendary by its nature, making it that much harder to know]
