Depends on who owns the property and from whose income was the same acquired. If it was acquired from the joint family income or by the brothers jointly, then the sale proceeds can be shared by all brothers equally.
Yes, you are right. However, if the father is a Hindu and at the time of his death, if his mother is alive, then she would also have a share in the estate of her such son and accordingly, the father's share in the property would devolve on the mother, wife and the children of such deceased man.
Nomination does not bestow title and as such the title to the property will be governed by the relevant laws of succession depending upon your religion. However, all legal heirs can together execute a family arrangement interse deciding to share the proceeds from the sale of the flat in such manner as they may deem fit.
If the share of the parties in the flat has been divided between them, by your father by way of some document, then your uncle cannot claim 50% in the property after 25 years from the date of execution of such document and his claim would be barred by limitation itself.
All legal heirs of the grandfather will be entitled to their shares in the property depending on the religion you follow, the share of the elder son who has expired will devolve on his heirs.