How to buy a ancestoral property registered in my uncle name as part of division

No.  It is his property, he can do as he wishes.

Pre-emption law, also known as a pre-emption right or first option to buy, gives a person the right to buy property before anyone else. It can be used in many different types of agreements, including land transactions, corporate agreements, and commercial agreements.

Since the property was an ancestral property and was partitioned among the siblings and one of the siblings is trying to sell the same to outsider then the other sibiing can make a claim for purchasing the same.

The buyer can purchase the land at a price set by the landowner or at market value. If the buyer doesn't accept the offer, the landowner can sell the property to someone else.

The buyer can match a genuine offer made by a third party to buy the property. The landowner must accept the matched offer.

The property is purchased at a price determined by a third party. 

Essential elemens for pre-emption:

He must have full ownership of that immovable property. There must be sale of the immovable property and it must not belong to the pre-emptor. There should be a certain relationship between the seller and the pre-emptor

Right of pre-emption does not apply in every case.  The right is enforceable among Muslims, but among Hindus, the right is applicable if so provided by the statute, custom or contract. It is not applicable in the following among other cases -

• Where all parties are Hindu and there is no relevant custom present.
• Where the vendor and the vendee are Hindu, the pre-emptor is a Muslim.
• Where the vendor and the vendee are Muslim, the pre-emptor is Hindu.
• Where vendor and pre-emptor are Hindu, the vendee is Muslim.
• Where the vendor and pre-emptor are Muslim, vendee Hindu.
• Where the vendor is Muslim, vendee and pre-emptor are Hindus.