Spica is the name of a star and it's what they call the imoutos in this. My guess is that the author likes Strawberry Panic and is referencing it by using that term. St. Spica is the name of the school from Strawberry Panic that won the Etoile Competition (French for star). The competition was a contest to see which couple had the strongest sisterly bond and the main characters were sort of assigned to each other like that. I can't think of any other reason they would use Spica. Since outside of being the name of a star it's not even a word. But I checked into the raws of strawberry panic and it's the same katakana being used.