There is no restraint in the SAstras that Son-in-Law's father should not be alive in case he has to perform the last rites of the father-in-law. The kartrutvam by Son-in-Law comes later in the order of preference of the eligible KartAs. Amongst the list of eligible KartAs, dauhitra ( son of daughter) ranks next to Putra (Son) followed by brother, brother's son and so on. The SAstras compassionately allow sagotra (One belonging to the same Gotra) or sakhA (one's friend) or a BrAhmaNa ( by giving some money) also to perform the last rites if there are no relatives available. The moot point is that samskAra of the dead should be performed .
It is not correct to state that those who have father never enter crematorium nor should they perform any funeral rites with the exception of their mother. For example, the dauhitran ( daughter's son) can perform the last rites of his grandfather, even though his own father is alive.
It is not correct to state that those who have father never enter crematorium nor should they perform any funeral rites with the exception of their mother. For example, the dauhitran ( daughter's son) can perform the last rites of his grandfather, even though his own father is alive.