How many treatments needed to get rid of head lice on boy?

My whole family is meeting for a vacation later this week. My sister just called to tell me her son has lice. She said it was a mild case and she treated it last night and spent 2 hours combing through his hair to get the nits. I have a 1 and 3 year old myself and have decided to no longer stay in the same house and my family rented our own condo.. it's caused her to be very mad because she says its already been taken care of blah blah blah... How many treatments can it take to get rid of lice?? We are already staying at our own place but I am still worried about my kids even coming near her 5 year old son?? I cannot find answers on the web do you have any insight??