Angst Dissonance: The reader is supposed to feel bad for Ariana, especially in the first year, when she has a secret to keep from her friends and is scared of losing them because of it. When it turns out her secret is that she is related to Sirius and that she has been repeatedly bullied or thrown out of schools because of this (no matter how unrealistic that last part is), the reader is left to wonder why this was such a problem. The fact that she whined about it on an almost constant basis, definitely didn't help. And then there's the amount of Wangst it turned into.
Crosses the Line Twice: Malfoy attempted to rape Ariana in her fourth year. While it's supposed to be a horrible scene, the bad writing makes it sound like Malfoy has about five arms in different places at once and Ariana is bent forward over her table, while her upper body is twisted by 180°, so that it faces Malfoy. Less squicky rape, more hilarious anatomy fails.
Damsel Scrappy: You could make a drinking game out of the multiple times Ariana gets kidnapped and needs to be rescued.
Fridge Horror: In this universe, Snape's loving parents were killed by werewolves. Sirius knows this; he was, in fact, right there when Snape found out. Remember that whole "try to get Remus to kill Snape" prank? This fic somehow made Sirius look even worse for doing that, though the writer doesn't seem to have realized it.
Ron the Death Eater: Ron himself is a bit dumber, but mostly spared. No, Draco gets hit with this worst, being turned from a relatively ineffectual jerk who eventually realized he wasn't cut out for the big leagues, to a pedophilic near-rapist.
Squick: Draco Malfoy lusting after and attempting to molest/rape a first-year.
The fact that she has a consensual relationship with fifteen-year-old Neville is pretty messed-up too.
The fact that Ariana performs basically a striptease to her bathing suit when she's 14 years old. All while being drunk and it excusing that.
Strangled by the Red String: Ariana just sort of tells Ron that he's in love with Hermione (and blackmails him into telling her), even though the two had no real romantic interaction prior to this. (They went to a dance together, but that's basically a throwaway line.) Harry's attraction to Ginny has slightly more build-up, but still starts mostly just with Harry making vaguely lecherous comments about her.
Unintentionally Unsympathetic: Ariana's mother, Tabitha. In the spin-off oneshot, Tabby Cat, all she does is visit Sirius in Azkaban to rant at him. The reader is allegedly supposed to sympathize with her, but the way she complains about how being his sister has made her life a "living hell" even though he has it much worse than she does doesn't make her too pitiable. It doesn't help that her choice of words implies she abandoned him without question the moment she found out he might have been responsible for the Potters', Pettigrew's, and the Muggles' deaths, never mind that he was her brother she had apparently loved, making her look like a Fair Weather Friend.
Worse, she had basically disowned him before that: they were best friends as children, but when he went off to Hogwarts and she turned out to be a Squib she basically cut him out of her life. Sirius himself tries to put a sympathetic spin on his, saying she was "hurt" to realize she had no magic, but the fact is she spent the last decade of her life blaming her innocent brother for her situation.