ANSWER:
You claim that her deen and Iman is weak but what seems to me that you have a really serious problem due to your ignorance! You want to change the religion of Allah and impose hijab on her from her own son in law? What is next? Maybe tomorrow you are going to order her to wear the hijab at home from you and your brothers as well!
The son in law is a permanent mahram for the mother in law for life. Even if the daughter dies or they get divorced, the son in law remains mahram to his mother in law!
You must seek at least the basics about Islamic knowledge before shooting rules on people from your gun of ignorance!