Wouldn't a tsundere nee-chan be weird? Though I'm afraid I'll never actually write tsuntsun moments for you, so there's that. There are too many hardships when it comes to writing about how we locked away some super powerful character and why, given her strength and the fact we've got brainwashing magic, we're bothering to lock her away in the first place, so I just made your sister a weakling. As far as powerlevels go, you should be able to destroy a whole city in today's world with one use of yours, by the way. Compared to that, Nimbus would be building-level. Able to down a skyscraper 'n all. Yuki has a bunch of gimmicks and no fighting skills. Only person in the world that can cast teleport, knowns mind-reading, can turn invisible and is apt at brainwashing - Brainwashing magic is a hard, arduous ritual that can take from days to months, depending on the will and power of the person being mind controlled, by the way. Though suggestion magic is significantly easier and can achieve similar effects. Aaand that's enough infodumping for now.