I think that's just an indication of how much of a retcon this really is - there was no reason to assume spike ever lived with the Sparkles given his hatching was immediately prior to Twi joining 'tia's school, Shiny (and cadance) were retconned in at the end of S2 themselves, and spike as "kid brother" isn't consistant with how Twi has treated him in earlier episodes, and his occasional fears (when faced with Owlysius or the fear doorway, to give examples) that he is to be replaced and sent away.
I like this new portrayal and that Spike is getting recognition as being an adoptive Sparkle - but it IS new, and not consistant.