1977’s The Luck of Brin’s Five is the first volume in Cherry Wilder’s hard-SF Torin trilogy.
Hard times have come to the family of weavers known as Brin’s Five. Death by starvation is a distinct possibility. Salvation comes in the form of a flaming object that crashes down into a nearby lake. Or rather, it comes in the form of the alien from deepest space who escapes from the plummeting space-plane.
Scott Gale is a human, part of a small scientific expedition examining Torin, the habitable (and inhabited) world of the 70 Ophiuchi system. His companions lack the means to retrieve him, as he was piloting the expedition’s only space-plane when it crashed. If he is to return to his on-planet base, it will be without any help from his fellow humans. To reach them, he must cross half a world.
To Brin’s Five, Scott (or Diver, as they nick-name him) is their Luck, a living token of good fortune. They welcome the peculiar placental mammal as though he were one of them, adopting him into their family. If Scott cannot return to his fellow humans, he is at least guaranteed a place with Brin’s Five.
To Great Elder Tiath Avran Pentroy, also known as Tiath Gargan (variously “Ropemaker,” “Lawmaker,” or “Strangler”), Scott is an unwelcome change. Why? Human technology is far in advance of anything available to Pentroy and his fellow grandees. Scott and what he knows could destabilize the entire social order. This is not a development to be welcomed … if you are already at the top of the social order.
Scott and the Five flee, but no matter where they go, the Strangler and his minions are close behind.
Rather than set her futuristic fable in an entirely imaginary stellar system, Wilder set The Luck of Brin’s Five in a nearby and very real system. The facts of the real system shape the story.
70 Ophiuchi is a binary star system just under seventeen light years away. Its orange suns are both less massive and dimmer than our own. Whether the system actually contains an Earth-mass world in the habitable zone is not known at this time but it seems a reasonable system in which to place an Earth-like world  (well, save for the quibble that the system is much younger than our own). I don’t know if the age of the system was known forty years ago. When this book was written .
Many SF novels take an essentialist approach, one in which family structure is defined purely by biological fact. Not Wilder. Her Moruians are marsupials but, as far as I can tell, this is not why each family has five parents (at least one female, at least two males). That particular solution to the issue of having enough adults around to raise children under often trying conditions is the one this particular culture has adopted, but it does not seem to be the only arrangement they could have used.
This book was a charming change of pace for me. The Moruians of Torin are alien and their social networks different from ours, but … they’re still essentially likeable. They are willing to adopt a perfect stranger into their families on the slenderest of pretexts. (It’s true the Strangler is not exactly nice. But he’s only killing to retain power, which is something we primates can certainly understand.)
1: The distance between the two stars varies from 11.4 to 34.8 AU. One wonders if worlds around one star or the other would experience significant climate variations driven by the apparent brightness of the other star?
2: The coincidence that the Solar and the 70 Ophiuchi systems both happen to have complex life pales beside the remarkable fact that they both host intelligent species and both species have attained impressive levels of technological development (close but not identical). This is not at all likely, but it’s a setup common to a lot of SF. Perhaps this is because Wilder cannot tell the story she wants to tell if Scott is marooned on a desolate Precambrian world of simple fungi and single-celled organisms.… 
3: For an amusing take on a similar concept, see this story, if you haven’t already read it.