When my mother broke her hip, she could barely walk and she was in incredible pain. She had fallen and cracked it. My mother-in-law was only having a bit of pain in her hip but when she went to the doctor, she was told she'd broken it and had to have surgery. How is that possible? My mother-in-law never fell or got hurt. She's active and plays tennis and stuff still. And, the doctor had to do special tests to see the break, he couldn't even see it on x-ray.
While it's not possible without seeing your mother and mother-in-law and their charts, it sounds like your mother had a traumatic hip fracture and it could be that your mother-in-law had a fatigue or stress fracture. In your mother's case, the break was sudden and harsh, and depending on where the location around the hip, may have caused a good deal of damage. If your mother-in-laws hip fracture is really a stress fracture, then it's not unusual at all that she didn't have a lot of pain nor could the doctor see it right away on an x-ray. This type of fracture has to be repaired just as much as the type your mother had because if it's not, the bones could move and cause damage. As well, the pain would likely increase, causing your mother-in-law to slow down.