In the ongoing case your father was plaintiff but he had given a power of attorney deed to a third person to conduct the case on his behalf.
However upon your father's death, the power of attorney becomes infructuous.
In the given situation you and other legal heirs of your deceased father have to file a petition under order 22 rule 3 CPC, for the substitution of legal representatives of a deceased party (specifically, the plaintiff) in a suit to prevent abatement.
It requires an application to be filed within a specified time, usually 90 days from the date of death, to bring the legal heirs on record. Failure to file this application within the stipulated period results in the suit abating, meaning it cannot continue.
The existing petitions that were filed before your father's death, will continue to be tried as per law.
No doubt the senior citizen plaintiff has died, but the case can continue in the same court.
As far as the flat jointly owned by your parents, your father's share in the property shall devolve on all his legal heirs equally, your mother's share will remain with her besides she will be entitled for a share out of yor deceased father's share in the property.