Hey there! c: I wouldn't at all mind critiquing both of your characters.
Her physical appearance.
There isn't too much information on your character's appearance aside from a pretty brief description. From what I can visualize, she doesn't sound theoretically bad, but might be too basic and simple because of that. There's no hue variation. Grey is a neutral color, but most shades of green and all blues are cold colors. This is gonna make it a little difficult for her to stand out, but it can be done. Keep in mind, though, that very few characters in Equestria have grey fur. It's a little difficult to believe she somehow managed to make it very long without being found given such a large difference in appearance to most other ponies.
Her cutiemark.
The colors you chose for her cutiemark stick out a lot against the rest of her design, which sounds like it would be mostly cool tones. It also sounds like it might be a bit too complicated when drawn out, because having several books and a gemstone is going to be very difficult to render in a simple way that only uses about 3 or 4 colors. If you want to keep the theme, I would suggest changing the crystal color and changing it to only one open book, rather than the concept you have now with one open book and two closed books. It's just too complicated and will be too difficult to execute well.
Her personality and bio.
Her personality and backstory aren't bad in the slightest, although I do think that they could use some further development. I feel like, the way you've built her condition, she really shouldn't be able to see perfectly in the dark. Maybe powering that down a bit by limiting it to just an advantage would be more rounded than what you have currently. I also don't understand what sort of supposed temptation overcame her? You specifically mention that she was ashamed of her cutiemark, and her having such a strong desire to return to something she views so negatively is unusual.
His physical appearance.
Admittedly, your second character sounds more appealing than your other one. Again, he has a very cold color scheme ontop of a very neutral color (white), but this is more plausible because white is actually a relatively common color in the setting you built the characters for. White tends to function better than grey because, although they're both neutral, white tends to have less of a "muddied" feel.
His cutiemark.
Once again, your cutiemark concept is far too complicated in order to be executed well. It also doesn't make much sense as a talent if it's entire focus is simply trying to locate his sister. Rather, give him something that represents naviation, with a simplified map or a compass star. It'll still hold a similar meaning, but will look better than what you currently have marked down.
His personality and bio.
His condition seems a bit unnecessary if your ultimate goal is just to give him a quirk involving the petting of his ears. Nerfing that entirely might make just as much sense, if not more, and the quirk wouldn't have to be changed at all because it holds a sentimental connotation. Otherwise, giving him an ultimate goal of finding his runaway sister makes more than perfect sense and I actually like that.
Overall, you've got a really good start on these characters. I think you should rethink their names, because they just don't make sense whatsoever in the setting. Starry has nothing to do with stars, and dark has nothing to do with anything dark. In fact, he's white, which makes it a little peculiar in the mlp setting.