Don’t forget that there are mangas of kid-demographic shows such as Doraemon, Yokai-Watch, Pokémon, Digimon, and others! Though, I might just be stating the obvious here...
I would also recommend Wonder Cat Kyuu-chan, because that series is a wholesome 4-koma. When my little cousin first began to use Mangadex, this was one of the first series I recommended. I view it as part of a “manga starter pack,” if you will.
Assuming that slice-of-life manga suggestions is what you’re looking for, I would also recommend Jahy-sama Won’t Be Discouraged, Chokotto Hime, and Sing, Erinna!
I hope that my suggestions are appropriate for your sister’s reading level, and that she is able to grasp the themes of each work, if you choose to show her any of these. Good luck!
P.S.) I was hesitant to recommend two of the most prominent slice-of-life manga, Aria (a.k.a. Aqua) and Yokohama Kaidashi Kikou (a.k.a. YKK), due to them possibly being middle-school reading level, but I’ll mention them here anyways just in case.