the only one method is:
1. never live with your mother or your mother in law.
2. never tell your mother all the things you will do with your mother in law. Similiarly, never tell your mother in law what you will do with your mother. Totally separate this 2 families.
Can you do that? If your mother ask you will your mother in law. You should answer 'I don't know, I haven't talk to her for a month.