The person in the next order must do the karma if the previous person in the order is not available or not eligible or not applicable or not willing to do etc.

The order is:
1. Elder son
2. If Twins, second born is considered as elder.
3. If sons for more than one wife, elder by age to be considered.
4. If get a male child after having a boy as sweekaram, own child
should be the kartha.
5. Son's son - Powthran
6. Powthran's son
7. A son who given up as sweekaram - Dhattan.
8. His (Dhattan's) son
9. Daughter's son (Tauhitran) having right in property.
10. Tauhitran (even if there is no property).
11. Brother if joint family continues.
12. Wife (Pathni)
13. If demised is a lady, if there is no issues, son of the co-wife
if any.
14. Husband (Bhartha)
In the case of a lady, for all mentioned relations should add "Husband's" as prefix.
15. Dhuhita - Daughter
16. Brother (Braatha )
17. Brother's sons
18. Asotara bhraatha - Sitappa, periyappa sons and their sons.
19. Pita - Father
20. Matha - Mother
21. Snusha - Daughter in law.
22. Powthri - Son's daughter
23. Dhowhitri - Daughter' daughter
24. Powthrasya pathni - Son's son's wife.
25. Their (24) daughter.
26. Sweekaram given (dhattan) son's wife.
27. Bhagini (elder or younger sister)
28. Bhagineyan (Marumaan, sister's son)
29. Sapindan (any one of our 7 generation pangali)
30. Samanodhagan (three generation from mother side)
31. Matru Sapindan (Mother's 7 generation pangali)
32. Mother's mother side three generation.
33. Jaamatha (Son in law) ?? !!
[This is called Dharma shastra ?!]
34. Sakha - any friend.
35. Dhanahaari - Any person who gets the property.

If many persons availble in the same category then elder by age should gets preference without checking the mother's side or father's side.