As others have said (more eloquently than I would have done), many of us here were equally avid readers at a similar age.
What I would add is that "Even if her 5yo sister is around, she often prefers to sit on the sofa and read and let the little one mind her own business" is far from grounds for concern in its own right. Three years is a huge age difference at that age and in fact the 8-year old may be escaping from the boring trivial boring and trivial younger sibling (my brother is 2 years younger than me and I knew the feeling well). That's not to say there's anything to worry about in their relationship, just that they don't have much common ground, and the younger is unlikely to join in with the reading - peace and quiet for the older. I see a similar attitude among those of my daughter's friends (same age as yours) with younger siblings.
The one caution I'll mention from my own experience decades ago, is that being bookish, intelligent, and well-behaved in school doesn't necessarily go down well with all the other children. It might be worth trying to gently find out if there are children who are being mean, even bullying, that she likes to hide from in her own little world. An increase in escapism (if that's what all this reading is) could indicate a minor falling out with a friend, which can feel pretty devastating at that age.