The property belongs to both the husband and wife. After his demise the husband's share would be divided amongst his family members ie there would be 3 shares. 2 children and wife.

Daughter of mother does not have any share in the property.

1) on his demise  his 50 per cent share in property would devolve on his mother , wife , 2 sons

On mother demise her share would devolve on her children equally 

No if the property is owned jointly then after his demise out of his 50% share in the property of the deceased will be equally divided among 4 (wife mother 2 kids), 

Or if thr person has any will or codicil it will be owned by the person named in the will. 

Mother's daughter doesn't hold any right on the property 

You have stated that the property has been registered on both the names jointly hence both the husband and wife have equal rights and share in the property.

In the event of intestate death of the husband, his share of property shall devolve equally on his wife, children and his mother.

The daughter of the mother of the husband cannot claim any rights in the property that belongs to her brother and her sister in law.

1. The co-owners are man and his wife.

2. Unless he makes a will during his lifetime, his share in the flat will devolve through intestate succession equally on all his Class 1 heirs (widow, mother and children).

3. Sister is not a Class 1 heir to a Hindu male who dies intestate.

Dear Sir

As per the latest Indian Succession Act as well as Hindu Succession Act

Wife, Daughter, Mother and 2 sons own a legal right in the property after his demise.

Thank you,

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-    Husband have only 50% share.

50% share is directly going to wife as she is co-owner.

-    Out of 50% share of husband, all 4 legal heirs will get equal share.

So wife, mother, and 2 sons will get equal share.

-    So mother will get 12.5% share.

Daughter of mother means sister have no right in this property.

But mother can  gift her share to daughter.

- Since, the property in the joint name , then each having equal right 

- After the demise of husband , his half share in the property would be devolved upon his all legal heirs i.e. his mother ,  wife and two sons equally. 

- No, daughter of mother not having any right even after the death .