Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine and obtain a history, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
I don’t have enough detail on the nature of the numbness but from the distribution of the numbness it can be due to a problem in the brain, spinal cord or the peripheral nerves. You say that she has suffered from a mild stroke? I am not sure what the location of the stroke is, but strokes in the brain in certain locations like the thalamus or the internal capsule can cause numbness in that distribution. If there is weakness as well, then other locations are possible. Sometimes, a person does not recover 100% sensation after a stroke, so this numbness might just be residual (if it was the stroke in the first place which caused the numbness). Certain medications like gabapentin may occasionally help with the numbness but not always. In any case residual numbness from a stroke is not indicative of anything serious.
Other lesions which can give rise to that numbness distribution may be located in the spinal cord. If associated with weakness then the spinal roots and peripheral nerves are potential causes too.
I would recommend evaluation with a neurologist to evaluate the numbness so he/she can examine your mother in-law and give you an answer
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck