It is most likely that she had osteoarthritis of the knee joints that is causing let knees to bend slightly. At this point in the absence of pain it would be best to control weight gain by dieting and plenty of walking. Encourage your mom to take plenty of been vegetables and fruits and reduce her intake of rice and sugar. If pain arises you may have to take her to an ortho surgeon who can examine her knees and order xrays if necessary.

Hello there. I can understand your concern. She is over weight by atleast 40 KGS, that puts immense pressure on the legs when she walks, and it requires more energy to walk for such people. Ask her to consult a orthopedician for the issue of the leg bending. Ask her to take a tablet. Neurobion forte daily once after food. She will have to lose weight, therefore take her to a good dietician. Best regards. Dr. Anil Kumar Jain.