Chicken wire works well, or even those little wire arched borders. I have found that even a small deterrent will keep the chickens away if they have other places to roam.
It is easier to keep chickens out than in, so fencing off areas you do not want them to go is more effective than keeping them locked in areas you want them to stay.
Good luck. Just tell your sister-in-law she better give you a steady supply of eggs every week for your troubles!!!