It's not going to turn into a harem, the imouto and the dark elf are just there to force her to be more proactive and admit her own feelings. Notice how even the chapters where they show up focus more on the female knight's feelings than in theirs.
what's really happened is elf lady has forced knight lady into compulsory use of her services and our guy reads elf lady as being as "wonderful person"
I like this so far, but what it really needs is a proper story, like a quest to go on or an evil villain, something to broaden the characters and the world. Just something other than boy compliments knight and knight gets embarrassed.
Well I mean her slowly being killed off by an amalgamation of curses that would only continue to be built up as she fights more wouldn't exactly be the best...