1) You should immediately seek partition by serving a legal notice to this effect on your mother and brother, calling upon them to arrive at a partition settlement with you. 

2) I'm prepared to serve such a legal notice upon them as your advocate. 

3) If they agree, then I will draft the partition deed, which will be executed by all the parties to the settlement and the property will mutate accordingly. 

4) However, if they Don't agree to arrive at a partition settlement with you, or if they don't respond to the legal notice, we will be filing a partition suit in district civil court. 

5) I'm prepared and willing to handle your case as your advocate. However, I require an exhaustive consultation session with you first. So pay a visit.

Mother can give her share to him. After father`s intestate death, you along with mother and brother inherited 1/3rd share each. So mother is free to give her 1/3rd share to him.

on father demise mother , you and your siblings have equal share in land 

2) mother can only bequeath her share in land to younger son by gift deed 

If it is self acquired property of your father, all three of you will have share.

Issue notice asking for partition of the same to your mother and brother.

Followed by suit for parititon.

If your father has died without any Will then all his properties including this land is liable for division among his legal heirs in equal share.

So in this land our mother having only 1/3rd undivided share , she can not transfer more than her 1/3rd share to your brother.

If she does then it can not challenged nor you can stop her from doping this .

However of you wish you can buy her 1/3rd share if she so agrees.

After the death of your father, you, your brother and mother are co-owners i.e. 1/3 rd share each.  Your mother cannot sell her undivided 1/3 rd share to younger brother. However, she may relinquish her share in favour of your brother and he can become the owner of 2/3 rd share.  Then  your brother has to partition the same with you according to his share.

She can only register her share if there is legal partition of the said property. Otherwise she will require your consent

The transfer shall be voidable  because in absence of any testamentary disposition the property of your father shall devolve upon all legal heirs equally. So can file a suit to declare the transfer null and void.

after death of your father all legal heir class1 will get equal share. your mother can gift her share without your consent to your brother. 

Mother can register only his share or if she got property by will then she can register same otherwise she cannot register it in name of younger son. Though for any property which is in her name she can register it to whom so ever see wish.

If the proeprty belonged to your deceased father and he is reported to have died intestate, then the same shall devolve equally on all his legal heirs including you.

Hence your mother cannot transfer the entire proeprty to your brother's name, you also have a right over the proeprty to your legitimate share in it.

If they dont listen to your request then you may file a partition suit and also an application seeking injunction restraining them from alienating or encumbering the proeprty in any manner till the disposal of the main suit for partition.

1. As per the information mentioned in the present query, makes it clear that he property was in your father’s name which means it was either self or ancestral.
2. In case of ancestral one, you all would have equal shares and you can stop her by filing suit for partition with application for injunction from creating any third party interest or from transferring on to younger son name.
3. In case of self one, if will is there then you can challenge the will and with the same application as mentioned above.
4. If no will, then file a suit for partition as per Hindu Succession Act with the same application.