Alignment means very little. You are assuming Sirius, but a more logical interpretation is that the structure is aligned to the position of the sun at dawn on Hathor's birthday, which is about 10 days earlier than the heliacal rising of Sirius. The actual difference in the position of the sun at sunrise on both those days is very slight and well within the margin of error.
Since it's Hathor's temple, this makes far more sense. Had they intended to worship Sirius, they would not have denigrated the deity by constructing and dedicating a temple to another god and never mentioning the main god whose worship was to be held there. Temple complexes do have structures to deities other than the main one, but all the themes and iconography at Dendera are not that of Sirius.
There are quite a few chapels dedicated to other deities... none of them are Sirius: [
www.touregypt.net]
In order to prove that it was Sirius, you have to eliminate all other possibilities -- you have to, for instance, conclusively rule out Hathor's birthday and show that all over the land, Hathor was replaced by Sirius. In order to do THAT, you have to show that historically Sirius was worshiped as an important deity (major temples, etc)
-- Byrd
Moderator, Hall of Ma'at