1) Provide the family tree and you both siblings are nominee of the property.

2) you can mention in society and municipal corporation and with sub- registrar.

See the flat first have to be mutated in name of the legal heris of grandmother since father is no more you along siblings and mother shall inherit same.

Further file an application for mutation before the muncipal authroity and you and your brother can relinquish your share in favour of mother through a registered gift / relinquishment deed.

On being complete owner mother can file nominations with the society.

For this, all the other legal heirs (you and your brother)  have to execute a Registered Relinquishment Deed relinquishing their respective shares in the property in favour of your mother. The Relinquishment Deed will involve very small stamp duty and registration fee.

If she was nominee in the society records automaticaly she will be registered as member and owner. You need to submit death crtificate to the society for the same. Once she becomes the member she can kep your brother and you as nominee

1. As per the information mentioned in the present query, makes it clear that now the property stands vested in the favour of all of you in the absence of any Will.
2. You all will have to apply for the succession certificate in the civil court of law which may take few months.
3. Once you have Succession Certificate then you can make an application before the registrar’s office for relinquishing your rights in he property in the name of your mother and get it registered.
4. You can mention in the relinquishment deed that after the demise of your mother the property would come to you both irrespective of the fact that whether there is any Will or not?

apply to society to transfer flat in mother name 

2) enclose grandmother , father death certificate 

3) your and  brother NOC 

4) society may transfer flat in mother name on execution of indemnity bond wherein you indemnify society in case any claims are made 

Your GM was co owner of 50% share

After he demise, provided she did not leave behind any Will, her estate will devolve on her legal heirs

Her legal heirs would husband and children

So if your grandfather is not alive i. e he died before your GM, then your father alone gets the share of your GM, as he has no siblings

For this you will need Letters of Administration from court and registered transfer deed made in favour of your father

Alternatively the transfer deed can be made im favour of your mother with consent of your father

Or if tranafer deed is made in favour of father then later on he can make a gift deed to your mother which attracts nominal stamp duty of Rs 500

Also transfer deed made by administrator under the LA petition in favour of your father will attract nominal stamp duty 

Once there is a registered transfer deed, you can submit that to society which can then mutate share certificate in your mother's name

Your mother then has to simply submit nomination form appointing you and your brother as nominee 

The procedure for obtaining LA from court is about 6-8 months and is an expensive process. There is no other alternative or option apart from that to confer proper ownership title on your mother

Mere transfer in share certificate on basis of NOCs is not a valid transfer and doesn't confer proper title

You may face issues in future at time of selling if NOC transfer is done in favour of mother

Only LA with transfer deed in favour of mother will confer clear title on mother

After death of your father property mutated to his legal heirs ,by which property is distributed to you your brother and mother

Apply to society for transfer the property with the death certificates of your grand mother and father

If you want to make your mother sole owner first you both need to relinquish your share to your mother by this your mother will become sole owner of said property, after your mother apply to nominate both of you

If it was your paternal grandmother who was a the joint owner along with your mother, then upon her intestate death the her share of proeprty shall devolve on her own legal heirs, i.e., your father. 

Now since your father is also not living, his share of proeprty shall devolve equally on his own legal heirs i,.e., your mother, you and your siblings.

As you want your mother to be made as an absolute owner of the property, you and your brother can execute a registered release deed relinquishing your rights in the property  in her favor after which she shall become the absolute owner of the entire property.

This is my response to you:

1. Since you also get a share, you will have to submit a release deed in favour of your mother;

2. You can also obtain letters of administration from high court;

3. Once the release deed is registered then your mother becomes the sole owner;

4. Then she can make a Will in you and your brother's favour;

5. Then your mother can submit nomination form to the society;

6. Consult a local lawyer and take steps.