Understand your situation. Take your parents or in-laws to travel when you have chance, make them happy daily.
For the letter to the Visa officer, it could be a one page letter:
1) Enhance the strong tie your mother-in-law has in china, e.g. other children and grandchildren
2) your finacial capability: support her living and bording, medical....your salary, company, ...
3) Purpose she comes, visiting....
Make the letter fun and sincere.
Good luck!