Though this is my first play through of the game, I knew of Est's presence in Chapter 18. Since I'm trying to get Gaiden chapters, I figured it'd be more convenient to kill her. Then I thought "Wouldn't it be funny if I killed Est with Catria?", so that's exactly what I did. She got a Level up from killing Est, too. It was pretty good as well, giving Catria HP, Str, and Skl (It might be worth noting her Spd is already capped). I think it's safe to say that's the best way Est could've contributed to my team. Also, props to being the only game I know of that rewards sororicide.