If i support my brothers / sisters children, but have no children of my own, genetic data of mine survives into future generations, after all I share half my genetic data with my siblings.

If my helping these children gives them a competitive advantage in further reproduction, where the total reproductive strength of these well supported kids is greater than the total reproductive strength my family would have if I had my own children as well and split our resources, then this behaviour outcompetes simple reproduction, and if this behaviour is genetically encoded, becomes a dominant trait over time.