Can I count on not breaking a bone if my mother never did?

No. Many factors have to be present for a bone to break. Only very abnormal bone will break without some sort of injury such as a fall. If your mother was fortunate enough to never break a bone, it does not mean her bones were unbreakable. It just means that she never fell hard enough to break her bone. It is possible that she had strong bones, & that may have been passed down to you.