In all non-human species selfless cooperation falls off a cliff beyond siblings, and AFAIU this comports well with Game Theory-type models for understanding genetics. Popular examples of non-human cooperation, naked mole rates and bonobos, actually live in communities dominated by sisters. (It's not often noted, though, in the breathless narratives extolling the virtues of cooperation and anthropomorphizing the rest of the animal kingdom.)

Human behavior, however, is still a deep, deep mystery in terms of evolutionary biology. I'm always wary of people applying evolutionary principles to human behaviors. Writ large you can see contours of what we would expect to see, but even then it's unclear why the boundaries are where they are, or to what degree we're projecting expectations into the data, etc. The speculation quotient is extreme. I wouldn't put any stock into evolutionary biology-based explanations for human behavior. And just as a practical matter, it's not like most people would leave their most hated cousin to die in a ditch; and though most people wouldn't leave anyone to die in a ditch--at least, if they knew that's what they were doing--I'm betting they're more likely to save a cousin than a stranger.

my viewpoint is that the human ability to cooperate effectively is why there's currently 8+ billion of us on earth and chimpanzees are an endangered species.

Our capacity for stories and language helps us create large cooperation networks, which is a unique evolutionary advantage.

Chimps have cooperation limited to "we are genetically close and you give me banana so I give you banana".

Humans can create something like the Roman Republic, or modern nation states and corporations, based on a shared set of stories and language (culture, also includes stuff like rituals, socio-sexual taboos, etc), which enables millions of us to collaborate together towards a common goal. Which is why we're so successful as a species.