More on Sirius's Gang of Slytherins

[olivierfouquet2000]

I will point again that if Slughorn was hosting three to four parties a year at Hogwarts at that 
time, if Snape was invited, and if older students like the Lestranges showed up each time or 
even regularly (all of this strongly suggested by canon), then Sirius would have be as justified 
to say that Snape was hanging out with a gang of Slytherins as Malfoy would be justified to 
say that Harry was hanging out with a gang of Weasleys (in fact even more so, Harrry sees the 
Weasleys about thrice a year). And that would be pretty well justified in my opinion. Especially 
since it is also strongly suggested that Sirius was invited to Slughorn's parties too, so he 
would have had first-hand evidence that Snape was hanging out with his not-so-dear cousin 
Bellatrix and her future husband.

JKR's timelines are notoriously fuzzy, but on that particular point, I see no terrible problem.
Olivier