While I thought that they got together a bit too quickly, I still appreciate them handling themselves in a somewhat mature way, conveying feelings to each other and not stumbling for 100 chapter with "oh no, I can't confess" stuff.
Many people didn't like them turning out to be siblings.
But I think it was a good plot point.
Them being related is an unsolvable circumstance rendering their love for each other impossible.
It's not something they can overcome, it's final and certain and it should be a painful realization both for the characters and the reader.
Thou it would have been better without the forshadowing from the "there are two keys" moment.
I enjoyed that On-chan and senpai were able to get together. That's nice that author didn't just drop them as is and their storyline as a whole was rather enjoyable while short.