You decide if you really can do anything, than you can most likely get away with changing a persons mind, so even if you do screw up you can still change something in their head to make them forget it happened. On this note you decide to try something small, insignificant, but what will it be? Of course! Her favorite color. If it doesn't work then you can just play it off as the set up for a gift.
You concentrate on changing the mind of your sister. You tap into her thoughts, which is a good start. You think about her favorite color, coming up with orange, deciding you want it to be green you focus on changing it. You think it worked, but there is only one way to find out. You ask her what her favorite color is.
"Green, why do you ask?"
"no reason."
It worked! With this ability you could do anything to her and she would be none the wiser. But mulling it over for a second you aren't entirely sure what you should do. You could always... no, you shouldn't. To be honest you had always had a crush on your sister, not always in a weird way at first, but over the years you realized that she symbolized everything you liked in a girl. You didn't like her sexually, you just though that she was a kind person, who deserved a kind person in return.
Thinking it over you could change it so you aren't biologically related... but would that be crossing the line? No, maybe it wouldn't be. After all who is around to know other than you. Nobody is getting hurt by this. So after deciding you take the leap, and make it so mentally, and physically you are no longer related in any way, opting to make yourself the adopted one. With that done all that was left is to decide on what was to be changed physically.
After what probably felt like a very long awkward stretch of time for anyone but you, you decide to generally increase her appeal. Your sister was never bad looking, still isn't, but she is pretty average. b-cup breasts, average waist, normal face, okay butt. Nothing especially excellent about her body except maybe her legs, which were especially toned after her running track for several years, but not something that is specifically sought after in a woman. You figure, with your sister rapidly reaching adulthood, you would do her a few favors for the future. First things first, the hair.
Her hair currently is a lot nicer orange, but you feel like it should be longer. You make her hair go down to mid back length, again making her not notice. Next you move on to the face. You make her cheekbones slightly more prominent and her lips ever, ever so slightly more pouty, barely enough to notice. Her eyes go from hazel to a deep green. And her nose gets slightly smaller.
Next you decide to go for the waist. She isn't necessarily chubby or overweight in any way, she just seems to be on the verge of it at all times. You decide to thin down her waist a little, as well as her upper body while you are at it, bringing her from 125 pounds to just over 120.
Thirdly, you target the legs and butt. first you add a bit more plump to the thigh area. next you thin the calves ever so slightly. and after that you decide to increase the butt size by a good amount, she had always been lacking a little in that department. You bring it out by almost a solid couple inches.
Last, but definitely not least, is the breasts. Sitting at a low b-cup she is average in that regard, but average seems a little cruel for what you have always thought is someone who deserves more. You decide to bring it up by a whole 2 cups bringing her up to a solid d-cup, nice and big without being a burden.
And with that concludes your sisters makeover session, all without gathering her attention and continuing to make it seem absolutely normal. And you gotta say, after all that, she is looking pretty good. Not to pat yourself on the back or anything but you seem to have done an excellent job still keeping that girl next door vibe, while also giving her enough plump everywhere to make her stand out from the crowd, and hopefully garner some attention. And judging from your tent, it already has garnered some from you. She used to be your sister you wager. 'used to be' being the key sentence here. Her new body and her recent lacking of actually being a relative is leaving you rather confused.