What can i do if my sister has mono, how do i avoid getting it?

Already exposed ?: If you had it as a kid (90%) you likely have immunity and do not need to worry. If you were exposed to her within the week before she was diagnosed, you had enough exposure and will just have to wait the 4-6 weeks it incubates to see if you got it. If shes been gone, any exposure over the 3 wks to 3 months of the illness may pass it. Good hand washing helps.