Attempt to murder and threat to do it again

Hello sir. I have perused your legal query. You have quite a few remedies available at your disposal. At the outset you can file a complaint for the offence of criminal intimidation which includes threat to life, property and reputation under section 506 IPC. You can also initiate proceedings under the Protection of Women from Domestic Violence Act. Most importantly you can initiate eviction proceedings under the Senior Citizens Maintenance and Welfare Act and the Rules made thereunder. Feel free to reach out to me in case you wish to discuss this in more detail.

Its best to report the attempt to murder case because not reporting the same would lead to fatal consequences.
You can also file FIR under Section 506 IPC.
However, if you don’t wish to, You can file an informatory petition before the Magistrate or give a statement in the nearest police station in writing that your sister is threatening to kill your parents.

this is a case where you need to file an FIR immediately so that the police can take appropriate action as per law. the offences mentioned by you are all cognizable offences and the police might need to take the acused in immediate custody to avoid commission of a serious offence.