[BykrBayb]

I can think of one other possible reason:

4) Sister has contracted an illness (possibly sexually transmitted) which she doesn't want to spread to her brother, and is too embarrassed to tell him about.

[sarasmom]

That would fall under #3.

[twinzmommy]

That was what I thought too -- any of the hepatitis strains, HIV exposure -- who knows? She may just be really, really embarassed.

I hate when the media only gives part of the story.