1. In case you were not heard by the Hon'ble Court, you can go on appeal for reviewing the judgement.

2. You can file a fresh case in a higher court by producing all the evidence like your sister's medical condition and her suffering from dementia since from a longtime, etc. and to declare the document she has signed before her death as not valid due to her health condition of suffering from dementia. 

3.  You have to prove that she was made to sign the document, without her knowing it of the consequences.

You can file suit to set aside the gift deed executed by your sister 

2) take the plea that sister was not mentally fit was suffering from dementia 

3) enclose her medical test reports which prove she had dementia 

You have good case on merits 

1. If you have enough evidences of your sister's medical condition as well as the alleged manipulation that happened during your sister's medical ordeal, then you can challenge the transaction in the competent jurisdictional court to set aside the document.

2. To gauge the case perfectly, documents have to be seen and then only we can come to the conclusion.

After the death of a very old person it is really difficult to challenge the medical cause of the death and the legal hairs automatically get transfer the property after the death of the person.

At the age of 81 it is really difficult to prove that your sister has any in cynical medical condition and medical condition for death is   cardiovascular.

 you need to speak to your advocate in case you really want to go for any legal action in this regard but please be aware that you may not be getting any relief in this case

Therefore the general rule on who can challenge a document is that, those persons whose rights, title or interest are directly affected by the document can challenge the document.You can challenge the execution of document with the help of medical records. If the lower court decreed against you, then file an appeal. Take a contention that your sister is not fit for executing a document. 

1. You say that you challenged a document of disposition of property executed by your sister, but your case was never heard. It is not possible that a civil suit filed by you was never heard.

2. Since you have already filed the case you cannot file a fresh lawsuit. 

3. Come with complete facts of the case as to the stage at which the case is pending in the court. 

See since you are not beneficiary and you have no right as such the other legal heirs can challange it though if there is any will or something previously made in your favour you can challange Same by filing civil suit.

Sir you don't have interest in the property of sister as you are not legal heir so there is no locus to your suit.

You should have proper medical documents to prove that she couldn't have any sanity at the time of signing papers. If you have then oppose the document in court. Regards GMGupta [deleted]

You can challenge the same in civil court. 

You have not informed which document you intend to challenge?

Is it a Will or some transfer document?

If your sister wasn't fully fit to understand anything how could she understand the contents of the property documents and sign them. This is a fraud and forgery has been committed. Challenge the documents and also lodge a criminal complaint against them.

Regards 

But there is an issue of inheritance. If your sisters son was the sole heir then he must the whole property by default. Nobody can interfere in the peaceful possession of the property. Therefore there was no reason for him to forge a document unless there are some other issues involved.

What was the case?

Whether you filed the medical records as evidence in your challenge?

What was the reason stated for dismissing the petition.

Did you prefer an appeal in this regard?

Revert with details for more proper opinion

You are not the legal heir to your deceased sister.

What's your grievances if her son is taking her property?

You may revert with details for more proper opinion.

Dear sir

There may be some discripency or misunderstanding of dates as there in no way court dismiss the case without hearing the plaintiff if he present before court.

You should again check the status of your case what is going on why there is delay in hearing or what is the reason that your case is not heard till date. 

You have a strong case and you should contest the case as your sister was not in condition to make any document for transfer her property.