I agree, I would say there is a strong second order effect for parents to see their siblings to reproductive age. As more generations stack up, any longevity effects quickly become negligible, maybe with the exception of hiper-social species with a learned culture like ourselves. Since this development is very recent, I don't think it could affect such a fundamental thing like aging.

Per Google, our closest living relatives, chimps and gorillas, very rarely display grandparenting behavior and have comparable lifespans.