Do all legal heirs have to jointly apply for partition?

All the 4 sisters are first class legal heirs in the property as much as your mother is first class legal heir, and in the absence of any Will, the law of intestate shall apply and all have a right to 1/4th share in the property of your grandfather.

As per the facts mentioned by you, the succession certificate will not be required / able to get you and others their respective legal share in the property. Your mother has to file a partition suit seeking her share before a civil court and in the case, she has to make all the legal heirs as defendants. 

The legal heirs who cannot appear the court can appoint a power of attorney of their choice to protect their rights. The property will be divided into 4 parts and 3 sisters will get 3 shares of 1/4th each and the two children of the deceased get 1/8th share each.

Please also note that any sister can file a case for partition of the property by metes and bounds and in case the property cannot be divided, the Hon'ble Court can order the sale of the property.

Kindly contact us for any further details and action, if needed.