It could be very distressing to receive someone else's 23andMe results, and confront one's mother about not being related. It also feels like an invasion of someone else's privacy to receive their data, and have your own data given to someone else.

I assume a genealogy company like this would write something in the ToS absolving them of any liability if they make mistakes, but would consider reading the fine print, and inquiring about a refund or new test kit. I can't see how they would be responsible for the incest - that's on the siblings.