Settlement cannot be revoked unilaterally except if executed by fraud/undue influence or if it is conditional settlement.

It is Completely valid as she is well within right to gave her share of the property to someone else out of free will.

Mother can execute gift deed of her one fourth share in property 

2) I presume gift deed was duly stamped and registered 

3) your mother has to file suit to set aside gift deed 

4) it  cannot  be revoked unilaterally 

After the death of your father who had not left any Will, his self-acquired property devolved equally upon your mother and the three siblings, by intestate succession. Therefore, the settlement of her undivided share by your mother in favour of one of the siblings is legally valid. It cannot be revoked now, if it was done by means of a registered document. The sibling who is the beneficiary, may transfer it back to your mother by way of a gift deed, which has to be duly registered. Afterwards, your mother can redistribute her undivided share.

Hi, the mother can only transfer her part of share ..The property will be inherited equally among all the legal heirs 

1. Your mother has settled only her share, which she was always at liberty to do.

2. She was under no obligation to consult her excluded  children before settling her share in favour of one child. 

3. It cannot be unilaterally revoked by her unless the deed of transfer is a conditional deed. If it is an unconditional transfer deed then only remedy in her hands is to file a petition in Senior Citizens Tribunal to assail the transfer on the ground that her son has neglected to maintain her subsequent to transfer, which she will have to prove.

See firstly the wife is absolute owner of property if she has settled through registered gift or settlement deed then in that case the settlement is valid and cannot be revoked.

Your mother is fully legally entitled to disposed of her share as per her own wish by a will or other testamentary disposition and can revoke it any time except in case of gift only.

If property is self acquired by  father then all legal heirs have equal rights in said property by filing partition suit you can claim your share. 

The mother is also a shareholder in that property hence she has got full rights to transfer her share of property to anyone of her choice, the other shareholders cannot object to this nor they can claim any share out of her share ion the property as a right.

She may not be able to revoke the registered settlement deed if it is not with any condition.

She can challenge the same in court and file a suit for cancellation of Settlement deed if any

consent of siblings is not required for mother to execute gift deed for her share of property 

She has transferred only her share so no NOC from them is requirer.

If the siblings are alive then their children need not have to sign anywhere to alienate the proeprty by the siblings on their own.

The children of the siblings do not have any rights in the property at least not during the lifetime of their fathers

There is no need for the other two siblings to sign.

Legal heirs consent required to sell or transfer. 

1. The property was of self acquired nature of your father, leaving no room for ancestral rights in it for may coparscner like you.
2. She has all right to give her share to anyone, but subject to the condition that her release of share must be in the divided property not undivided as this may harm the rights and life of the other share holders.
3. She can also revoke it, if it has not been registered or if registered then there must be some condition to be full filed.
4. Consent of all persona having share in it, is must before releasing or selling her share to anyone.

Yes other siblings need to sign or relinquish their rights

No, mother is at absolute liberty to give her share to any. Children have no say.