I really like this chapter.  In the long run, the description of Harry living with Sirius is probably realistic.  If Sirius hadn't died at the end of OOTP, and if Harry had been able to live with him, a similar scenario might have developed.  Sirius bitter, drinking too much, having difficulties with Harry not actually being like James, not really parenting Harry very well...  I like the way you had Harry dream Snape's visit, clearly showing how Harry appreciated Snape's actions and care for him.

That was a perfect way of having Harry come to see reality better.  And I like Snape's response to Harry when he "lost it" in his anger, frustration, and grief.  Excellent chapter.

I'm curious about where you're going.  I assumed at first that you were just working through the summer.  But now I'm wondering if you'll keep this going into the next school year.  It's really developed very well and could go on at length if you wanted. 

There are lots of abused-Harry-mentored-by-Snape stories, but yours is far more compelling than almost all of them.  I think having Snape so bitter and tending toward malicious, without actually acting on the maliciousness is an excellent "take" on the character.  It's more like he wishes he could be malicious, but other aspects of his character (honor, convictions, some sort of hidden empathy?) keep him from following his temptations.  So Snape is very IC.  Harry, of course, is not so IC, or he wouldn't be in Slytherin, right?  This is a bit AU in that regard, but that's okay. 

 You write very well.  Your angsty parts are really quite moving and you make me feel along with Snape, which is very satisfying.  Hope you keep this story up.  I notice most of your stories are rather short.  This one would make a great longer story if you wanted to keep going.