@CashCowBob What's so wrong about that? When you're watching a documentary about a deer and you see the deer escaping from a tiger, you root for the deer. But if you're watching a documentary about a tiger and you see that same video, you'll be rooting for the tiger then.
It's the same theory here. It's just a story. Don't be hung up on what's good or bad. At least it's not hypocritical and trying to justify everything the MC does. The MC is not a good person here nor a bad person, the sister is the same...neither good nor bad (well, at this point at least...later she does murder the mother in cold blood and frames the MC so she turns bad afterwards).
Anyway, this story is purely for entertainment purposes. Revenge stories usually give readers a satisfying feeling so it's popular.
Let her suffer, let her feel that everything was unfair and she didn't deserve it, let her die not knowing why she died. Some of us here enjoy that sort of stuff. This story is definitely directed towards us. The main character is bordering on being an anti-hero sort of character so what's the problem? (Only reason she's probably not is because the opposing party here is also kinda bad.) Can't be you haven't read anything with anti-hero protagonists now can it? Well, those are quite something. I recommend you read those...or don't, seeing how you're reacting to this.
Also, the thing about her being a child....well, children need to disappear from this world anyway. They're annoying and stupid. She'll grow up eventually so don't mind it. To us she's still more or less the bitch from the future so it doesn't really hit your conscience much to want her to suffer. Who knows? We also might just be fucked up. Don't forget, we the viewers are not the manifestation of morals either. We've got lolicon degenerates here. Who do you think you're targeting your words towards?