Divorce notice to my sister from her husband

Good morning sir, I'm from Telangana state, my sister is fat and she got married with her husband in 2003 and they are having a male child who's 10 years old, they were happily living their married life when they were in town, due to some financial crisis they shifted to Hyderabad in 2008 soon after getting out of financial crisis my brother in law started to harass my sister for two reason that she's fat and another that he got overwhelmed by the fashion of city girls and he want my sister to loose weight on my father's expenses, my father and I spend about 3 lakhs for her treatment at different clinics and hospitals to loose weight but i didnt worked out, now my father in law have different plans of getting married to a young girl who's not fat and he has send a notice to my sister regarding divorce and has said orally that he'll give the child to my sister and with a monthly compensation of 3000 Rupees to live life in Hyderabad, the rent only costs 4000 rupees excluding other expenses to live and she's earning around 3000 by tailoring, please help me to how to hit back to him with assistance of a lawyer. Thank you