In the religious texts, it is written that those blessed with a son need never worry about their last rites. Just by being available to be seen at the time death the son becomes free of parental debt. One whose son offers shraddh goes to heaven. The last rites by the son protect the father or mother from birth in low places.
According to Dharm Raj, if one has several sons the elder one should perform the last rites. The other sons may be present, but must not perform the rites.
If there is no son, the wife/husband should perform it. If they are not available, then a real brother should do it. In their absence, the son-in-law or his son can conduct the ceremony.