I have worked with kids between the ages of 2 and 19 for the last 15 years, and have a couple suggestions that have often worked...
People, no matter their age, seem to respond well to games. Kids are no different, and can often get their exchange in around the house if it is presented in game format. What do they enjoy right now, and what do they do to get it? If they enjoy watching t.v., playing video games, having friends over, going to the movies, but they do nothing around the house to earn that, their exchange is out.
If she takes the time to sit down with the three of them and work out rewards and penalties, and what they need to do to help around the house and earn rewards, she may find that they will start doing things without being forced. She gets them in on the rewards and penalties that are to be used, because then when its enforced they can't "blame her", they themselves helped set them. For example, you need 50 points or "stars" to earn $10 for the movies or whatever. You earn points by doing various housework, get them taken away for poor school performance, backtalking your mom, etc. It may not be an overnight success, but I guarantee you that anyone would be interested in a game at their level, you just have to find the one that works for them.