Harmless and beautiful is right. What difference does their being brothers make? Will one of them get pregnant and have a deformed baby? Uh... no. If they are brothers, then it would have been a lot riskier if Pim had let Gino push him into a relationship with Sabrina.
When Marcella joined Gino's and Pim's hands right before she died, she was giving her blessing to their relationship. The fact that she did not include Sabrina in that blessing implies that what she was blessing was not their relationship as brothers but as lovers - else why exclude Sabrina?
Regardless of what the book says, I'm not sure the movie makes them brothers. It wouldn't be the first time a movie changed something in the book it was based on, and since this forum is for the movie, not the book, what matters here is the movie, not the book. I can see evidence in the movie both for and against their being brothers, but the evidence that they are not brothers seems much stronger.
AGAINST their being brothers:
For one thing, the movie makes it pretty clear that Étienne is Gino and Sabrina's father. They have the same name (Étienne and Stefano are the French and Italian versions of the same name, which in English is Stephen; since Marcella probably is Italian, since her name is Italian, she may have called Étienne by the Italian version of his name, Stefano - while the native Belgians would have used the French version, Étienne). And if the photo Marcella shows them is not of Luk Wyns as a young man, they found a photograph of somebody who looks too much like him for it to be a coincidence. That Étienne is Gino and Sabrina's father is almost inescapable in the movie.
But Yvette told Pim it was a one-time thing when she and Étienne had sex at the hotel in Lille, and she bit him when he tried it again, after which they evidently broke up. If the night in Lille was a one-time thing (and whatever else she may be, Yvette isn't the sort to lie about something like that), then Étienne is not Pim's father. And he really looks nothing at all like Étienne or like Gino and Sabrina.
In casting the roles, they did an uncanny job of finding actors who look like siblings to play Gino and Sabrina; they could just as easily have found a boy who looked like them to play Pim, or at least dyed Jelle Florizoone's hair a darker color to make the connection more believable.
Pim and Gino don't look alike - at all - so Sabrina can't have been thinking they were brothers when she looked at them immediately after seeing her father's picture for the first time. It's much more likely that she was looking at them to see if they also recognized that it was Étienne.
Unlike an earlier poster, I didn't think the photo of Stefano looked like Pim at all, but it did look very much like a younger version of Étienne. That Marcella had hidden the photo from her kids until then because she didn't want them to know that Étienne was their father makes much more sense than that she didn't want them to know that Pim was their brother, since the photograph looks a lot like Étienne but almost nothing like Pim.
Anyway, why would she not want them to know it if Pim was their brother? It makes sense that she wouldn't want them to know Étienne was their father, but she loved Pim, she strongly approved of the relationship between Pim and Gino - so why would she try to hide their being brothers from them? It makes no sense. And, as Judge Judy says, if it don't make sense it probably ain't true.
FOR their being brothers:
The only thing I see is the fact that Étienne gave both Gino and Pim a knife. That could be seen as a father giving both of his sons the knife he thinks all boys should have. I can't think of any other evidence that Étienne is Pim's father - in the movie, which is the only thing I think we should be talking about here. What happens in the book is irrelevant since Defurne may have changed things from the book. But Étienne may have given them the knives because they were the only boys he was close to at all, and for whatever reason boys' having knives was important to him. That is at least as good an explanation for the knives as that both boys were his sons.
The bottom line:
I'm going to say it again, because it's important: What the book says is irrelevant. What the movie shows is what matters. The movie makes a much stronger case that Gino and Pim are not brothers than that they are. The "well-kept secret" - in the movie, at least - is that Étienne, Gino's boss, is also his and Sabrina's father, not that Pim is their brother. One makes sense and is supported by evidence in the movie; the other doesn't make sense because there is no reason why Marcella would keep that a secret; the matching knives are the only things in the movie that support that theory, and they can be explained in other ways.
Since I'm sure there are still people thinking, "But the book... ", I'll say this one more time: We are talking here about the movie, not about the book. What the book says is no more important than what any other book says unless the movie shows the same thing. The movie doesn't show definitively that Pim is Gino's brother, but it shows considerable evidence that he's not.
Why does it matter?
As I said at the top, it doesn't. They're lovers, in an extraordinarily loving and tender and beautiful and sexy relationship; that's what matters. Whether they're also brothers doesn't matter at all.
I'm just trying to get straight what the movie is showing about it, and that's what I come up with after having watched it many times, paying special attention to this question. I put a frame showing Stefano's photo right beside a frame showing Étienne and another showing Pim; the photo looks a lot like Étienne and nothing like Pim.
My personal preference would have Pim and Gino be brothers, because I happen to think it's neat for brothers to be lovers too, but the movie just doesn't support that scenario; if Defurne intended for it to, he failed.
Incest is a tricky subject. Normally, my disgust is with the fact that they were raised together as brothers. I don't care even if it's not blood, if you have sex with your adopted sibling who you were raised with in a family since you were small, that's messed up to me. But that's not the case here, they were always friends, but they were not raised as a family. So it's a lot more of a grey area. When it comes down to it, I have to admit, It's just gross in my opinion to have a sexual/romantic relationship with a blood relative. If I was in a situation where my kid fell in love with a blood relative, even if they never met until adulthood, I would probably react the way homophobic people do to their gay kids. Give me a gay kid, a polyamist kid, let them date people older than me once their adults, but incest across the line as far as I'm concerned.
Now that I am older and wiser I realize that we have no control over whom we fall in love with.