Evolution question about white fronted bee eaters.

In the white-fronted bee-eaters, sons often help rearing their siblings instead of dispersing and reproducing on their own. Fathers will also often harass their sons who are trying to raise their own offspring to force them to help rearing their siblings instead of their own offspring. Use kin selection theory to explain:
1- Why would sons rear their sibling instead of producing their own offspring?
2- Why do fathers harass their sons who are trying to raise their grandchildren?


I know it has something to do with inclusive fitness and altruistic behavior.