The Chronicles of Narnia and Harry Potter are MUCH more appropriate for an 8yo than the Twilight series.
Narnia deals with violence to some degree, but its' all from the POV of children. In every single book, there's at least one prepubescent human character. HP also deals with some violence, and it gets rather "dark" by the end of the series, but it's still from the POV of a teenager. The romance doesn't get any further than "snogging", even when marriage is being discussed.
In Twilight, there's sexual tension throughout the whole series. In the first book, it's a typical high school romance (except that it's a vampire she falls in love with). Then there's some more romance, broken hearts, choosing which boy to date, etc, stuff in the later books- but the first 3 don't get into these things any more than Harry Potter. But, some of the "thirst for blood" stuff is definitely more graphic than anything in HP or Narnia. Both show violence and killing, but not in the "your scent is so tempting it makes me want to drink your blood" kind of way.
But the 4th book goes into somewhat graphic detail of a marraige being consummated. I'm OK with my 12yo reading it, in large part because of the context (ie: wedding night, not just boyfriend/girlfriend) but I don't think I'd have let any of my kids read this before starting puberty. There's also a somewhat violent aspect to the sex that doesn't (or anyway shouldn't) exist in real life. With my 12yo and 14yos, I'm ready to discuss the ways in which this portrayal of sex differs from IRL sex, and explain what aspects of the descriptions in the book would make for an unhealthy relationship IRL. This isn't something I'd be comfortable discussing with an 8yo.
There's also some pretty whacky pregnancy stuff (that goes above and beyond "real life" pregnancy and birth) that IMO needs to be countered with a lot of "this is what normal pg and birth are like for normal humans" to avoid imbedding a fear of birth into her psyche. And some descriptions of vampire children that, while outwardly not really any more graphic than something like Dementors, could make children feel especially vulnerable while reading it.
ETA: if you're looking for more books to recomend to her, I'd suggest The Hobbit and then the Lord of the Rings books. The writing is a bit harder than HP and Narnia, but there's nothing innapropriate for children in any of the books. There is violence (and some fairly graphic descriptions of Medeival war techniques) but overall it's about Good fighting Evil, lots of action and adventure, etc.