Inter-Lazio in Coppa Italia quarterfinals will be played at the San Siro. There will be no draw to determine the venue of this match because both clubs are seeded. And in that case, Inter are ranked higher than Lazio and thus the club of Spalletti will get to play at home.
If it were Inter-Novara or Lazio-Benevento, then there would be a draw which is only needed for seeded against non-seeded teams.
Regarding Milan, the Rossoneri will play the quarterfinals away if the two games were to be on the same day. Despite having a lower ranking, Inter have the right to play at home since the Nerazzurri finished in front of Milan last season in Serie A. That establishes the regulation of the Coppa Italia and so Napoli-Milan will be at the San Paolo.