Just one thing that I must point out is that you located this story in the wrong category. This should be in the Harry Centric or in the Misc area as there is almost NO Snape here, and definitely does not classify as a Snape mentors Harry story.
Sorry if it sounds harsh, it's just that I was up all night reading it and waiting for some Snape/Harry interaction and was disapointed that there was none.
Other than being located in t he wrong category I really liked your story. It is well written, the plot and the characters well developed, and Sirius talking from behind the veil to Harry was original.