She should be able to easily cross her legs at the ankles. If you are talking about laying your ankle on top of your other knee then unless your mother is thin and very flexible this is probably not a good idea. Doing so could place a great deal of stress from the weight of her leg against the end of the appliance of her KR and possibly slowly loosen her knee appliance over time. She might could do it briefly but this is probably not a good idea of making this a regular habit. Her orthopedist or his nurse could tell you for sure.