Alright. I don't think it takes Sherlock Holmes to be able to read the signs that Alya's sister is actually the girl that Masachika talked with as a kid. Let's look at the signs:
- Looking at her locket of Sa-kun, we see a picture of a kid whose face is obscured by glare, but whose hair looks suspiciously Masachika-like (also sa is one of the kana used in his name, so sa-kun very well could have been a Russian-style nickname like Alya is)
- When she meets Masachika for the first time this episode, her brain stops functioning for a bit as he clearly was recognizable to her in some way.
- When she hugged him in the tea store, it sparked a nostalgic, comforting feeling in Masachika.
This is setting up to be a love triangle with a pair of sisters. Also, this airs on the same day of the week as that other show that features a love triangle with a pair of sisters. Coinicidence? I think not!
Anyway, we got some more background on how Suou and Masachika ended up separated. Still not sure why they are keeping their familial relationship a secret, but we might get there before too long. Also of note this episode, the fan service was completely absent compared to the past two.