In the queried scenario, if the property was gifted (according to Shari’ee requirements) or sold by your great grandfather to his 2 sons and it is for this reason that it was on their name then they will be considered as the owners of the property. In this instance, the property will be distributed amongst the heirs of the 2 sons only. On the other hand, if the property belonged to your late grandfather until his very end and it was merely legally registered (on official documents) on the 2 sons’ name (without it being sold or gifted to them), then it will belong to the heirs of all 4 of his children. There is the distinct possibility that the norm in India is that after the demise of a property-owner, the property is officially registered under the name/s of the original owner’s male heirs and not his female heirs. If this is the case, then the mere fact that the property was put on the 2 sons’ names will not make them the sole owners. This matter should be clarified with some seniors who are acquainted with the modus operandi of these matters in India or who may have been witnesses to the gift or sale transaction (if this scenario was perhaps applicable).
If the heirs of the 2 sons and the 2 daughters agree that the property should devolve on the heirs of all 4 siblings (son and daughters), then there is no dispute at all and the property can go to the heirs of all 4 siblings. However, if the heirs of the 2 sons (based on the registration of the property) are assuming sole-ownership and the daughters’ heirs are contesting the sole ownership of the 2 sons, then they (the 2 daughters’ heirs) will be required to produce credible witnesses who testify that the property belonged to their grandfather until his very end and the 2 sons were merely registered as owners of the property on paper. If they fail to produce such testimony, then the heirs of the 2 sons will be required to take a Qasm (oath) that the property did not belong to the grandfather, rather it belonged to the sons in which case the property will be distributed amongst the heirs of the sons only (and not the heirs of the daughters). This procedure (of producing witnesses or taking an oath) must however be carried out before a Muslim judge or in the absence of a Muslim judge, before a Mufti.
From the tone of your query, it appears that the heirs of all 4 siblings have no objection to acknowledging joint ownership in the property for all the 4 siblings; in which case there would be no need to produce witnesses or take an oath from any party. This would be a mutual compromise and the advisable route to take in order to maintain peace and family ties amongst the family members.