1) consent of sister is not required if it was self acquired property of father and he executed gift deed in favour of son 

2) ask father to act as witness to sale deed 

Yes if his rights are affected then he can challenge it

- As per Section 122 of the Transfer of Property Act, Gift is the transfer of certain existing movable or immovable property made voluntarily and without consideration, by one person called the donor, to another person called the donee, and accepted by or on behalf of the donee.

- Hence, for a Gift deed, Acceptance is mandatory by the donee , and such acceptance must be made during the lifetime of the donor , and if the donee dies before acceptance, the gift becomes invalid. 

- Further, after the acceptance by the donee , gift deed cannot be cancelled unless it is a conditional gift.

- Further, Gifts that involve immovable property should be registered under the Transfer of Property Act, 

- Hence, if the said gift deed is registered from the office of the registrar , and further the property already transferred to his son , then that gift deed cannot be cancelled on any grounds. 

- Further, as the said property was self acquired property of his father , then his father was having his right to gift that property to son without taking consent of his sister . 

- You can proceed to purchase that property , however suggested to make witness to his father. 

Based on your narration, you can go ahead and get the property registered in your favour, without sister's and father's signatures. However this should not be construed as legal opinion. 

Are yo buying the proeprty that belongs to all the children of the father?

The father has already transferred his rights in the property to his so by a registered settlement deed, thereafter which the son has become an absolute owner with clear and marketable title to the property.

In such a circumstance why at all his sister, i.e., a third person to the property need to consent for this sale of property by the son in favor of the prospective buyer?
However you may obtain a legal opinion from a local lawyer and take a decision as per the opinion rendered 

If a gift deed is executed by the donor without any influence, force or coercion, then it cannot be challenged or disputed by any person.

Dear Sir,

once gift is always gifted property.  Thus father can't cancel the gift deed without the consent of Son because it was executed by the father. The consent of the sister is not necessary and if she challenges then u can just contest the case and her suit will be dismissed. The law is as follows:

Cancelling a Gift:

Section 126 provides two modes of revocation of Gift:-

1. Revocation by mutual agreement:- A gift may be revoked upon the happening of an event which is not dependent on the donor where the donor and donee may agree. The revocation must be in express form and not of a wish or desire.
   Revocation by rescission:- The gift is a transfer of ownership without any exchange of money or any consideration which is made voluntarily. So, where there is no free consent of the parties, the gift can be revoked or if it is obtained by coercion, undue influence or fraud then the gift is voidable.

CASE

“S. Sarojini Amma v. Velayudhan Pillai Sreekumar 2018 SCC OnLine SC 2200”

“In this case, the Supreme Court observed that there was no completed gift of the property which is in question to the respondent by the appellant and the appellant had right in cancelling the deed.”

Conclusion:

The Transfer of Property Act, 1882 lays down provisions relating to gift deed and cancellation of the gift deed. Any person who is the real owner of the property can transfer his or her property to anyone by way of gift. A gift deed actually says that any person who is the owner of the property can transfer the property to another person without any consideration. Similarly, there are provisions for cancellation of the gift deed as well.

Dear client,  

It is always advisable to have all parties involved in the gift deed present and consenting to any further sale or transfer of the property, especially when there are siblings involved. If the son is hesitant to involve his sister in the sale of the property, it may be a red flag that there are potential issues or disputes between the siblings that may cause problems in the future.

It is important to ensure that the gift deed is legally valid and that the property can be transferred to you without any legal complications. You should hire a lawyer to verify the validity of the gift deed and conduct a title search to make sure that the property is free from any encumbrances or liens.

If the gift deed is found to be legally valid and there are no other legal issues with the property, then it may be possible to proceed with the purchase. However, it is always best to have all parties present and consenting to avoid any potential disputes or legal issues in the future.

As the father has gifted his self-acquired property to his son, the son's title is legally absolute and exclusive. There is no need for the daughter to consent to the proposed sale. However, please have all the documents examined by a competent lawyer before proceeding further.