Not to defend NH, but them (and other anti-Sakura fans) seeing Sakura as an "abusive" person is a legit argument. You have to remember not everyone researched the definition of "tsundere" or have limited taste in manga (my sister, for one, has only ever read and watch Naruto, nothing else) to where they will not see NS as a common trope you would see if you read more manga. Then you have people who hate the trope point blank. Not everyone tolerates Sakura hitting Naruto; for me I was uncomfortable with part 1 Sakura. Especially when her reason for hitting Naruto were hypocritical (like the peeing moment in the FoD). And Kishi was aware of it as well, if you see from part 2 you can easily see Sakura soften around Naruto.
I don't know why Sakura ended up taking the most crap for her "tsundere", maybe because Naruto is popular. In general, "tsundere" is becoming a less liked character trope. My sis doesn't hates NH, but she hates Sakura's personality more which is why she shrugged when NH was canon.
The other thing fans (NH fans especially) conveniently forget about the 'abusive' trope stuff is that it was all done in the framework of Part 1...when they are all crappy, selfish characters. The fans who want to keep Sakura in the abuser/tsundere role are very selective about keeping Sakura this way, while letting Naruto, Sasuke and even Hinata grow out of their Part 1 characters and develop.
It's a very one-sided, Sakura-bashing type of idea.
That doesn't mean that people (like your sis) can't be uncomfortable with the idea of Sakura hitting Naruto. No more than they might be uncomfortable with the idea of the main character taking a leak in the middle of a suspenseful scene. (Imagine Superman or Batman stopping to take a whiz.) My point is some of those things — the rough exterior that hides a soft side (Sakura), potty humor to break up tension (Naruto) — are exaggerated for the sake of the story. And also for Japanese manga humor and tropes. (It may seem unsettling for a new reader, but both those things have been done a lot, thus they became a little less unusual.)
So if someone says they don't like Sakura because she's abusive, make them dig a little deeper. Because if Sakura has to remain abusive, then Naruto has to remain the self-centered (and often mean) prankster, and Sasuke has to remain too cool for everyone. And Hinata has to forever remain weird.
If Sakura is still abusive, but Naruto has turned into Naruto the beloved hero, Hinata is the pretty, self-assured girl, and Sasuke is the new non-pervy Jiraiya out to save the village....then you know it's more about hatred of Sakura than it is about what Naruto was ever really about.
edit: Just thinking about it, I think that it also matters if you read the manga or watched the anime. I think they play up a lot more of that Tsundere crap with Sakura in the anime and sympathize more with Hinata. So if you're watching Sakura in the anime...you're probably getting a much more exaggerated version than the manga.