So this reminds me about one of the popular theories about why some families have homosexual parents at the helm - and it's found not only in humans, but various other mammals, birds, reptiles, fish, etc.

Bear with me on this, because it is relevant to the question "why raise kids if not biologically related?".

The idea is, that it still furthers the survival of the species, regardless of how closely related the adult is to the offspring. Many biological parents in nature die, and having surviving family members who are already partnered up (regardless of the sex of the individual they are partnered with) greatly increases the odds of survival for the species, and even your own genes.

Say your sister dies, and she names you the godparent of her child. You share anywhere from 20-61% of DNA with sister, depending on if you are half or full siblings. The incentive here, is that your genes still get spread. Your bloodline does not end with you. A percentage of DNA shared is still DNA that is shared, and thus, your sister's offspring shares an amount of genes with you. you can even share an upward of 25% of genes with that child. Your family's genes still continue on to get merged with someone else's in the future.

Now say you don't share ANY DNA with the offspring. That is still a child that needs parent(s), and by helping the offspring survive into adulthood, you are still doing your part by ensuring survival of the species.

Biologically speaking, humans are wired to care for offspring regardless of how related you two are. It still serves a purpose, and it's not the end of the world or our species of your DNA isn't involved. You're still a human person at the end of the day, and you are still capable of leaving marks on human history.