A few weeks ago, billboards began sprouting up around Orlando, Florida, with advertisements for Hank Green’s first novel An Absolutely Remarkable Thing. On the face of it, this was not such a remarkable thing. After all, Green is a local boy and, being one half of popular YouTube channel Vlogbrothers (3.1 million subscribers, 711m views), it could be expected that his publishers might shell out for marketing. Except this was all paid for by his own brother: young adult novelist (and the other Vlogbrother) John Green – and just one part of John’s larger effort to promote Hank’s debut across the globe. (Among others, a professional women’s Frisbee team in Texas, AFC Wimbledon, the Netherlands’ national quidditch team and a Glaswegian rugby team were also suddenly festooned with a new literary sponsor.)

None of this is remarkable if you know the Greens; it’s more a fitting escalation of a long history of public displays by two brothers who have made careers from their interactions online for the last decade. To rewatch their early outings is both charming and disconcerting: here are two young men unknowingly on the verge of becoming very famous for their shared smarts, for their sincere speeches about self-identifying as nerdy, and for their affectionate ribbing.

But the internet was a very different place in those early videos, and it seems a long time since either brother had the freedom and the anonymity to film themselves killing 15 hours in their local Target. These days – John now 41 and Hank 38 – both are far more likely to be found earnestly delivering lectures on the benefits of gay marriage to millions of viewers and generally extolling the virtues of being nice. John is now famous for his emotionally wrenching bestsellers, while Hank is best known for his various YouTube channels – Crash Course, SciShow, The Lizzie Bennet Diaries. But now he has his own book. An Absolutely Remarkable Thing is a comic take on first-contact stories that follows 23-year-old April as she gets a crash course in negotiating online fame when her video of an alien robot’s arrival on Earth goes viral. It’s no polemic, but this is a book that is trying to teach its readers lessons hard-learned.

“I definitely don’t think April is me or anything,” Green says. “But there are some things I didn’t know how to say without fiction. And everyone wants to write a book, right? Isn’t it the coolest possible thing to do?” Down the phone from a hotel room in Australia, he sounds distracted. It transpires that he’s editing a video while doing the interview, all the while preparing for Melbourne’s branch of VidCon, the world’s biggest convention for YouTubers – which he also founded. (His website reads, drily: “I’m Hank, I do a bunch of stuff.”)

For Green, reading is “one of the times in my day that I never regret”. His favourites include Michael Crichton’s Jurassic Park, Kim Stanley Robinson’s Mars series, Frank Herbert’s Dune and Orson Scott Card’s Ender’s Game. But An Absolutely Remarkable Thing is perhaps most like a cheerier John Wyndham: a first contact story that, rather than dwelling too much on why aliens have arrived, focuses on how humans react.

Was writing it everything he expected? “No-oooo?” he warbles. “Well, I thought it would be more magical.” He sounds slightly morose. “But talking to authors, this isn’t weird – it’s not like I did something wrong. It’s not like the muse strikes and the words flow and, 5,000 words later, you have an outline and know what you are doing. The only thing that gets writing done is time.”

There are many differences between Green and April. For one, he was older than her when he became famous online and he had a backup career as a biochemist; it also happened much more slowly. At VidCon he’s been “hanging out with 19-year-olds who have massive audiences and trying to remind them, ‘Views aren’t a measure of self-worth! You have 60 more years of life! You will do other things aside from this!’ I do try to push a message that isn’t too stodgy because some of them go, ‘Yeah, uh-huh, whatever.’ Boy, I hope they read this book. I hope they hear some of the things it says.”

While YouTube has brought the world many gifts in the form of tutorials and funny animals, its scale means that alongside the genuinely enlightening content is the depraved and ridiculous: conspiracy theorists and “alt-right” propaganda, “Evil Peppa Pig” videos, and even prank videos by parents who have since been arrested for abusing their children in the pursuit of fame. Even as someone sitting firmly on the lighter end of online content – spending an afternoon giving away marshmallows to strangers seems a long way from Evil Peppa Pig – Green seems to feel some responsibility for it all. “I was part of growing this thing that has been used differently [to] the way I had anticipated,” he says. “I felt this would be a good thing for society and I used all my power to make this thing powerful, good and strong. And then it turns out that it is a mixed bag. When the first three things people think of when you say ‘online video’ range from kinda gross to super gross, then I am so glad to have worked so hard to try and build a stable and happy place.”

Green’s book is already a bestseller across the world, and a sequel is in the works. But did he feel any pressure, writing in the wake of his brother’s success? “Ultimately, I knew even if it was a bad book, someone would publish it,” he says. It’s surprising to hear him voice such cynicism, but he’s pragmatic about his position. “It is a problem. The good news is that even if I’m a bad writer, I will probably get a book out there. But then critics will not like it and that won’t be fun. There was also part of me that worried people would say, ‘This is John Green’s brother and he has this big internet audience, let’s just publish the book and make a buck out of it.’ I think trusting people is a problem for anyone with an existing audience who wants to do another thing. But I didn’t worry that I would be overshadowed by John. Of course, I’ll be overshadowed by John! That is how it should be. It doesn’t bother me. It would be one thing if it was being overshadowed by my unsuccessful brother, but I think it is fine to be overshadowed by the author of The Fault in Our Stars.”

• An Absolutely Remarkable Thing by Hank Green is published by Trapeze.