1) You must add the buyers as "party respondents" in your appeal. Since they purchased the property during a pending lawsuit (Lis Pendens), they are bound by the final court decision

2)Application for Temporary Injunction (Order 39, Rules 1 & 2 of CPC): you

must make out a case that cancellation of settlement deed is illegal seek stay order restraining further sake and for setting aside sale  of property 

It is not clear whether the properties in dispute are self-earned assets of your father. Assuming them to be so, your father was well within his rights to cancel the earlier settlement and write a fresh one in favour of anyone of his choice. In that case, you cannot legally challenge it with any success. An ancestral property is one which has been in common possession and enjoyment of all co-owners continuously for four generations and has not been divided by any partition. Please provide all the relevant facts to get a proper advice.

You cannot stop her from alienating the property in any manner until there's a stay order by court restraining her from doing so.

Filing a fresh suit on the same cause of action is barred by law of res judicata hence you cannot file a fresh suit also.

You may have to pursue the appeal properly.

You need to seek injunction immediately in the same stating it will make your appeal infructous. Actually in lower court you should have first prayed for injunction 

Option 1: File a Temporary Stay Petition and Implead Buyers in the Existing Appeal (Your Lawyer's Advice)

This is the legally sound approach. Under Section 52 of the Transfer of Property Act, 1882, the doctrine of lis pendens applies - any property transferred during pending litigation is bound by the final decree. Your sister sold the properties while your appeal was live, meaning the buyers are "transferees pendente lite" who purchase at their own risk. Under Order I Rule 10 CPC, you can implead them as party respondents in the existing appeal. Simultaneously, you must file an application under Order XXXIX Rules 1 & 2 CPC seeking temporary injunction to restrain further construction and sale. The court examines three factors: prima facie case, balance of convenience, and irreparable injury. Construction causes irreversible harm to the disputed property, strengthening your case. The drawback is that arguments may pause during this process, but this is procedurally correct and avoids res judicata issues.

Option 2: File a Separate Suit (Your Friend's Relative's Advice)

This approach is problematic and likely to fail. Filing a fresh suit on the same cause of action is barred by the principle of res judicata under Section 11 CPC. Your appeal is already pending on the title dispute; a separate suit would be dismissed as duplicative. Additionally, Order II Rule 2 CPC requires a plaintiff to claim all reliefs arising from the same cause of action in one suit - splitting claims is prohibited. While a separate suit might seem faster, courts will view it as an abuse of process. The only scenario where a separate suit could work is if you seek a completely different relief (e.g., damages against the buyer), but for stopping construction and sale, the injunction must be sought in the pending appeal.

Recommendation: Follow your lawyer's advice. Implead the buyers in the existing appeal and file a urgent temporary injunction application. This preserves the lis pendens protection and ensures any further transactions remain subject to the final decree. The temporary delay in arguments is far less damaging than allowing construction to proceed unchecked.

At the outset, your matter is already at the appellate stage, and therefore your primary rights are being adjudicated in the appeal. The acts of your sister in selling properties and permitting construction during pendency of appeal are legally questionable and subject to the doctrine of lis pendens under Section 52 of the Transfer of Property Act. This means that any transfer made during the pendency of litigation is subject to the outcome of the case and does not create absolute rights in favour of the purchasers.

However, despite lis pendens, practical damage can still occur if construction continues or third-party interests are created. Therefore, immediate interim relief is essential.

Your lawyer’s advice is legally correct and strategically sound. You should file an application for temporary injunction/stay in the pending appeal itself seeking:

restraint on further construction,

restraint on alienation of remaining properties, and

status quo with respect to possession and nature of property.

Simultaneously, you should implead the subsequent purchasers as parties in the appeal. This is important because:

they are now necessary parties,

any order passed will bind them, and

it prevents them from later claiming independent rights.

It is also possible to seek urgent ad-interim relief (ex parte stay) to immediately stop ongoing construction, especially if you can show that irreparable loss will be caused.

The suggestion to file a separate suit is not advisable in your situation. Since the title dispute is already sub judice in appeal, a fresh suit may:

be objected to as parallel proceedings,

lead to conflicting orders, and

delay your overall remedy.

Courts generally discourage multiplicity of proceedings when effective relief can be granted in the existing matter.

As regards the ongoing construction, courts are usually inclined to restrain such activity because it changes the nature of the property and complicates final relief. Similarly, further sale of remaining properties can also be restrained through injunction.

In conclusion, you should immediately proceed with:

filing an injunction/stay application in the appeal,

seeking urgent interim relief against construction and sale, and

impleading all subsequent purchasers.

This is the most effective and legally appropriate course to protect your rights at this stage.

Since your appeal is already pending, the proper course is to file in the same appeal an application for interim injunction/status quo, implead both buyers as pendente lite purchasers, and seek an urgent ad-interim order to stop construction and any further sale; a fresh separate suit is generally not the better remedy. Under Section 52 TPA, the later sales remain subject to the appeal result, and the appellate court has power to grant the same interim protection as the trial court.

Next step you may consider

Ask your lawyer to move an urgent stay/status quo petition with impleadment and request early hearing with photos of ongoing construction and sale documents.

Sir/Madam,

it is the best option to file a temporary stay petition and implead both the buyers in the existing appeal. 

Dear Client, 

A completed registered transfer is ordinarily not cancelled just because the transferor later changes his mind. The sales made by your sister during the pending appeal are not the end of the matter but they do create an urgency. Section 52 of the Transfer of Property Act states that during pending case over immovable property, it cannot be dealt with so as to affect the rights of the other party under the decree. For stopping the construction and the further sales, you may file an urgent interlocutory application in the same appeal seeking status quo and restraint. Without the injunction order the appeal by itself will not freeze the property. You can also ask the court to implead both the buyers in the appeal. A lis pendens transferee is treated as a person claiming under the transferor and they can protect their interests. Order I Rule 10 lets the court add a party whose presence is necessary to completely decide the dispute. 

A later suit involving the same property and the same core issue can be subject to Section 10 CPC problem because the matter is in pending appeal. A separate suit is usually considered if it raises a genuinely new cause of action that the present appeal cannot cover. Hence, at this stage it is suggested to file the stay, implead the buyers and seek an order restraining further construction or registration over the remaining properties. I hope this helps. In case of queries, feel free to reach out to us. 

Thank You.