it's alright as long as they aren't related by blood that would be disgusting, they are just siblings by law so when the time comes they can get married. :p
Lol, if we go by the bible then we are all partaking in incest since we all start from Adam and Eve therefore we are all related. I personally don't care what two consenting adult or mature people do together. The only concern i might have is when they reproduce since the chances of the child having a genetic disorder dramatically increases but that concern is not enough for me to say or think siblings should not be together.
this kind if story always are a bit strange ... we can understand that they loved each other but now they are siblings ! How can we go out with our brother or sister ? Oo