Precautionary Police Complaint

Hello, My sister got married 6 months ago and moved into her in-laws place. Her mother-in-law is a widow so the 3 of them stay together. Things have started to take bitter turn for the last couple of months where my sister's mother-in-law is trying to create issues between the couple and is seen propagating bad things about my sister. Son's effort to stop this behavior of his mom has yielded no result. Few of the behavioral traits are 1. Wanting to sleep with the couple 2. Cooking n serving food only for the son, no food for my sister 3. Seeking constant attention 4. Not letting husband n wife travel anywhere over the weekend 5. In the middle of the night, asking the couple to leave the house 6. Threatening to commit suicide and write a note with their names Concerns 1. Both husband n wife are working in IT industry thus stay out of the house for 12-13 hours 2. Both have great understanding for each other and are supportive in such situations 3. Both are willing to move out but don't wana leave the elderly alone 4. They have tried to mend things with the lady 5. 3-4 days in a week they try to sleep together, still things haven't improved Even my interventions have failed miserably. I would like to take some actions to protect the newly wedded couple and safeguard their future in case of any mishaps like suicide. What actions should one take in such situations? Thanks in advance for your co-operation.