1. During the lifetime of your mother none of her children has any share in the property, movable or immovable, owned by her. As a corollary thereto, you cannot renounce a share which you do not have.
2. The renunciation of share can be done after her lifetime by executing a relinquishment deed or family settlement. If you wish to relinquish your share during the lifetime of your mother then it has to be preceded by a partition or family settlement between your mother and her heirs.
3. The relinquishment of share, subject to the above mentioned rider, will be legally binding on your brother. Get a flawless partition or family settlement deed, as the case may be, drafted by a lawyer to ensure that the subsequent relinquishment gets the legal character you wish to give to it.
4. Inheritance will not open if you relinquish your share.