I mention exalted, but I really try to avoid looking at RPGs for writing inspiration unless they're specifically meant for what I'm trying to write. Just a shorthand way for me to describe the sort of power some of the characters have. Still, Scion has piqued my interest in the past. I'll probably give it a look.
When they use their powers, it really just fatigues them like anything else. It's a high magic setting so everyone has SOME degree of magical talent and power. What makes the sisters special is that they have access to three different kinds of magic, their talents are very diverse, and they have much deeper wells of energy. When they use up their power they just get really tired. And like anyone who is fatigued mentally and physically, but still trying to surpass their limits, they tend to act irrationally and make stupid mistakes. For all their power, they're essentially human in mindset.
I've pretty much sketched out how the fight with the pirates will go. They use their green dust magic to create a "sea" for the ships to sail on ABOVE the waves, then the mist rolls in like a fog. The mist itself is dangerous to people caught in it, so my main has to fight them in the sky, separated from her forces below who have to deal with the stragglers who wade ashore from the non-flying ships. Up there, she ends up having to deal with cannon-bearing ships, the pirates themselves, and their powerful leader whose magic is producing the green dust. She's relatively new at fighting so all of this together is a severe challenge.
In another scene I wrote out, I tested the limits of their immortality. A traitor manages to get the drop on the other sister and runs her through the chest. She has powerful healing, but with the downside it brings all the pain of the healed injury at once. So she puts on a great show of humbling the traitor while discreetly cauterizing the wound. Allow me to show that exerpt:
General Yestrep leapt across the room with lightning trailing behind. Propelled by his wings, Aurora only had enough time to get one hand around the hilt of his short sword before he buried it in her chest. A slight gasp of surprise escaped her lips as blood stained her white robes.
"For Saratsin!" He hissed, "For all the lives you failed to save."
“Saratsin...your home," the princess whispered with blood at the corner of her mouth. Her eyes grew unfocused and closed.
"You could have done something," he shouted, "and for your continued failure to lead, I now take your heart."
He attempted to twist the blade, but found both of her gentle hands wrapped around his. Her grasp was soft, yet firm and inescapable as silk cord. He tried to pull free but her hands wrapped around the grip and pommel, completely trapping him.
"Your sword missed my heart," the princess said. Her eyes snapped open and filled with fire. "But only your sword."
Aurora rose from her gilt throne to her full height and left General Yestrep dangling an inch above the ground. He kicked at the princess and struck practiced blows, but to no apparent effect.
“Help me, you sandbags!” the general shouted at his guards. In response, Aurora flared to radiance. Her long multihued hair rose up as a dreadful mane and her eyes exploded with the full brightness of the sun. The traitor guards dropped their weapons and shielded their own eyes from her punishing light.
“Would you have had me crush the rebellion, General?" Aurora said "I could have given your people the horrors of a second war so soon after the last. Should I have retaliated against the griffons instead? Would they then be our allies today against the dragons?”
Her grip on the general’s hands tightened from silk cord to steel vise. Bones cracked. Joints popped. His wings beat against the air, but not a sound escaped his lips as he struggled in vain.
Aurora’s voice took on an echoing quality, her words clipped and precise. “You mistake mercy for weakness, restraint for inaction, deliberation for apathy. You think my great power gives me free reign to do as I wish.”
The smell of a crisp summer’s day turned to the smell of cooking meat, mixing with the metallic scent of Aurora’s blood. Light poured from between her fingers and starting with the hilt the sword grew red-hot, the heat slowly creeping up the blade towards the wound still staining her pale dress. The soldier turned red himself with effort as he pitted his will against the pain.
“You know not how much I wish that were the case,” she said in a whisper that nonetheless resounded through the room, “How very often I wish I could unleash my wrath on the deserving. But my power obligates me to be reasonable. To show mercy. To act truly noble and righteous, even to traitors. Do you understand me?”
“YES!” He cried out as his will broke. Smoke poured from the hands still wrapped around the white-hot sword. He writhed in agony and twisted away from the blade to no avail. “By your mother and father, yes! I understand! I’m sorry!”
With a flash of light, Princess Aurora released the seraph general. He tumbled down the dias from her throne and fell in a heap, trembling and holding his hands. He flexed his fingers and found them unmarked and unharmed, completely without burn or bruise. Yestrep looked up at the princess in her bloodsoaked robes, and his eyes fixed on the heat-warped steel buried between her breasts.
“Then, this day, you do not deserve my wrath,” she said, returning to her throne and only now glancing at her wound. “You deserve my pity.”
I have not finished writing the next scene yet, but that is the exact limit of her immortality. As soon as the coast is clear, she collapses, barely managing to keep herself alive until her sister returns to help her heal. If he actually had run her right through the heart, the ending would have been very different.