彼女達の「姉妹」とは。

冬イベで磯風をちゃっかりお迎えしてました。ついでにこの前９９になりました。育てる艦がいないので早く残りの陽炎型の実装をお願いします。

What is their meaning of "sisters".

I welcomed Isokaze during winter event; and finally raised her to level 99 recently. Since I don't have any ship to level up, I really hope the rest of Kagerou class would be implemented soon.