You already know this relationship is not normal, but who cares? That’s not what matters here.
I think you’re asking everyone because you sense the inevitability of change.
What matters now is how you feel about her. What do you want going forward?
Change will eventually happen in this relationship and you have two options for guiding the outcome:
Be honest with her about how you feel. Maybe she feels the same way? Maybe not? You guys need to talk about what you want and how you feel. You’ll own the outcome, for better or worse, but at least you’ll have taken the initiative and learned something in the process.
Continue with the status quo. This might buy you time and stability right now, but it will cost you later. If she meets someone new and exciting, expect the sleepovers to be the first sacrifice. If you’re really OK with this to keep her as a friend, that’s a valid choice, just be honest with what you want.
The cost of self-sacrifice is high and sometimes brings with it resentment. Being honest about what you want is the best shot you have at keeping the relationship.