Your Brother in law is using pan and smoking. Both are culturally acceptable. 6 years passed and your sister able to adjust with that, so definitely the issues for fight is not that alone. From one point of view you can't start treatment which usually fails in most cases. Let the couple to talk about their issues to a clinician/ good psychologist(NIMHANS). If the issues is rigidity in attitude this will go on. If ur brother is motivated to quit then it will work. What is the maintenance factor for substances use, let's clarify.
Next Steps
now suggest her to stop arguing with all issues. pen down their issues of good n bad aspects of their relationship. let them talk about each other in front of a therapist.
Health Tips
Definitely need a consultation if both of them feel in that way.