When I had mono the doc. told me that it's far, far, less likely to be transferred from the body once someone has it. Basically, don't worry about it at all.

I had it for basically 3 total weeks and made out with my gf regularly and she didn't get it.

Got mine in 3rd year of college. Went from 161 to 147 on a 6'0" frame in 10 days. Looked skeletal.

Had HORRIFIC recurring nightmares every night for over 2 weeks, and horrid lower backpains and headaches. MONO BLOWS. It doesn't PWN at ALL.

Be nice to your sister, she's probably going through hell right now. And doctors say it can last much longer than 3 weeks. It's random.