Hi S.,
I have to agree with Bobbie, throw away all diapers, put her in underwear everyday, use a rewards system, like everytime she goes, she gets to put a sticker on her potty so that she can see how many times she's accomplished using the potty (that's what I did) and like Bobbie said, make a huge deal out of it when she does go. My husband and I would do this really goofy "pee pee in the potty" song and dance that would make my daughter crack up laughing. Sometime I think she went just to see us look as foolish as we did because she got such kick out of it! But seriously, at her age, toss all diapers and say, here's the deal, use the potty or deal with the accidents. It's not going to be fun but I bet it will be QUICK!!
Truly Best of luck to your sister!
BTW, my daughter practically potty trained herself, she just decided one day she didn't want to wear diaper anymore, so having a 10 month old, don't let it scare you for one second. I was scared to death of how in the world I was going to get my daughter potty trained because I had heard the way you are "supposed" to do it is keep them in the house for about three days (sometimes preferably when they're sick) and then put them on the potty whether they have to go or not every 20 MINUTES!!!! I thought NO WAY am I going to be able to do that!! Well, I didn't have to, every child is different, and no one goes to college in diapers, so don't worry about it!
Take care,
S.