Imagine this, the sisters are exhausted and recovering from the intense fight they just had. The assassin, taking proper advantage of this moment of weakness -- -a moment of weakness they planned to use well in advance -- succeeds in getting Melida's blood. Well darn, game over. The assassin will escape and win. But! Elise here, she reveals her love for her sister has unlocked this recovery power. Great, now they can both fight again, and the goal becomes "Stop the assassin from escaping with my sister's blood". Now, the power isn't being used to solve any conflict or bad situation, it's being used to sustain the conflict by a reversal of fate. The good guys can still fight! Yeah! Suddenly, this terrible situation(that's still terrible) has been reversed by the powers of poetic justice, powers of poetic that have already been built up in this narrative! (Go sisterly love!)
Now we're braced for an epic fight scene where everyone is trying to stop the assassin from escaping. Furthermore, all of the logical explanations for the assassin's difficulties escaping in place are already set up:
There are teachers as well as other powerful people surrounding the dome, right? This means the assassin can't escape by ordinary means.
Why would they have trouble using their "unordinary" means of escape? The sisters are recovered and attacking them! There's no time!
Well, why not just kill the sisters? Direct orders from the Guild that the family is too powerful. Already built into the narrative!
And most importantly, why should the audience want to believe any of this? Elise-sama just reconciled with her sister, and you're telling me the story's going to end? No! Get the bad guy Elise-sama!
It sounds so epic now!!!
*Huff, Huff*
Everything about this situation just works me if the assassin is allowed for their plan to initially succeed(get the blood) before this powerreversal happens. I think in this case, the author pulled the reversal just a little too soon to maintain the tension. The difference is slight, but it's the difference between me going "Wow, awesome!" and "Meh, okay. That's interesting and isn't that a shounen power-up?".
Anyways, Regardless if I accept the development as a "power-up" or not, the current, written situation still leaves me questioning a bit. I just don't believe the sisters can last 5 seconds against something who: one, absolutely manhandled their senior who should be stronger than them; And two, is known as an super-elite assassin. But if there are obvious, environmentally-imposed restrictions on said assassin, and said assassin is fighting girls I'm heavily encouraged to root for(get that blood back!), I don't even have time to question. This is too awesome!