I… I have so many questions. Everything I commented last chapter is still up for debate, but even more prevalent. If it's scenario 1 though, I think she's gone well past the point of being cutely naive into the "she needs professional help" category.
Also I guess one more possible subscenario exists. Maybe she literally thinks (and possibly has been taught by her friend) that sibling is synonymous with lover, in which case I'm extremely concerned.
Welp, putting all that aside. Don't worry girl, you've already filled one of the most classic onee-chan archetypes, the tries but is hopeless type!