Rent refusal

Respected, We live in a tenanted house for past 30 years. Initially, the rent agreement and receipt was inthe name of my grandfather and then transferred to my fathers name. My father also died in 2012 leaving me and my mom alone. The rent was paid to the landlord who then died around 10 years back leaving behind her two heirs. Being a tenant we never knew to whom to pay the rent as both the siblings were not in talking terms with each other. The younger son who used to live in our building agreed that he would accept our rent and provided rent receipt as well. The elder daughter of landlord moved away to her in laws and had no conversation with us. We had been paying rent on time and also had a good relation with the son. Suddenly, around 6 months back the son also sold his part of flat and shifted to another location without our knowledge. Since then he has not accepted our rent. I had approached him over telephone but he is showing ignorance. I have tried sending over two money order's at his old residency address(I do not know his new address nor is he wiiling to tell) for last 4 months Oct15 to Jan16 but it has returned back with the PO statement mentioning "Receiver left without instruction". Need your kind suggestion and help as to how should I tackle this problem.