To everyone arguing whether this is incest or not, here's your answer...
Incest (/ˈɪnsɛst/ IN-sest) is human sexual activity between family members or close relatives. This typically includes sexual activity between people in consanguinity (blood relations), and sometimes those related by affinity (marriage or stepfamily), adoption, clan, or lineage. (Grabbed from Wikipedia. Found some other definitions but they technically mean the same thing)
So yeah, I guess they are incest by affinity because she is adopted and considered their sibling by law.
Just wanted to say, if you are bothered by the story or disturbed by the plot, stop making yourself miserable, just stop reading and stop judging others who are liking the story. We have our own separate lives, live yours and leave others' alone. I don't think this is too hard to do. ╮( ̄▽ ̄)╭
What's happening here? Haha