1.If the land is in the joint name of your brother and mother, the house constructed thereupon can not stand in the single name of your brother and it has to be in the joint name too. However,if the title of the land stands in joint names, then your mother can not sell the land without the consent or signature of your brother.
2. If title of a property flows without any interruption for 4 generations i.e. from great grandfather to great grandchildren without executing and registering any deed of conveyance, then the said property is called ancestral property.It appears that the instant property is not your ancestral property. Moreover, a minor's share of ancestral property can not be dealt with his/her mother and can not be given any No-Objection by her. District Judge only can permit dealing with the share of the minors property for his/her welfare. So, if the property i ancestral one, then you have your share in it which your mother can not convey to your brother for and on your behalf.