Commentary: ウルスラ「姉さま……（ギュッ」エーリカ「し、ししし姉妹でそういうの良くないんじゃないかなぁ……で、でもちょっとだけなら（ｺﾞﾆｮｺﾞﾆｮ」

Ursula: Nee-sama... *grab*
Erica: S-s-s-sisters shouldn't be doing that kind of thing, should they?.... b-but if it's just a little (mumble mumble)

I was torn on whether this qualified as yuri/twincest but I decided the commentary and image title (Ururika, i.e. a couple name) pushed it there. I can't imagine Erica blushing so much over holding hands, so I'm going to guess she intercepted a groping attempt or something.