I'm sorry, could you explain in a bit more detail? It sounds like what you're saying is that your sister's husband signed up for 23andMe and sent in his DNA sample. Then, 23andMe said "based on your DNA, we believe another of our members is likely your niece, here is her contact info...". Then, your sister's husband contacted this person (the potential niece), and after some chatting it seemed likely that your sister's husband's brother was indeed the girl's biological father, and this was then later confirmed with additional DNA testing.

If this is in fact the story, doesn't this seem like an invasion of privacy on the part of 23andMe?