It wasn't something I much focused on, since it wasn't really very necessary to the plot. It'll probably come up later, though, since theoretically even a younger vampire could control an older one if they manage to get close enough to force the exchange.

Tony or Celicia's blood could help sustain Steve, but the effect won't be as certain or as beneficial as Ezrabet's blood. If Tony were still alive-but-infected, his blood would hold a relatively pure strain of Ezrabet's infection that could be worked with. Since he's died and it's bonded with his own DNA, he's a direct descendant but it's not a perfect match. Going with the flu analogy, it would be like getting a vaccine made for last year's flu. Could work, might not.