Since the two heroes joined the Avengers, Hulk and Thor have become brothers in arms and have more often than not fought side-by-side against the forces of evil–though Marvel actually made them more than symbolic brothers bonded through combat, but literal brothers, and it did so by stretching the limits of its own established canon to its absolute limits.
Thor’s family tree has always been a bit complicated especially when it comes to the identity of his mother. Many casual Marvel fans would recognize Thor’s mother as Frigga, though that isn’t actually the case. As has been established for years in Marvel lore, Thor’s father is Odin and his mother is Gaea, goddess of the earth, though a recent development has stated that Thor’s mother is actually the cosmic entity known as the Phoenix. However, Gaea was the canonical mother of Thor when Hulk was supposedly added to the family, making their connection a bit more clear at the time while also remaining incredibly muddled and downright confusing.
In Giant-Size Incredible Hulk #1 by Roger Stern, Zach Howard, and Cory Hamscher, Hulk encounters Earth Mother Gaea who believes the Hulk is the latest iteration of Earth’s ancient race of giants. It is revealed that giants are Gaea’s children, meaning that the Hulk is directly related to Gaea according to the supposed revelation that he is, in fact, of giant descent. Gaea describes Hulk as the newest, most perfect, and most beloved of her own children.
Whether the Hulk is actually a giant or not, the fact that Gaea recognizes him as one of her children means that he is the long lost brother of Thor. The revelation of Hulk’s supposed lineage is a truly bizarre metaphorical connection between Hulk and Thor without fully committing to the idea. Hulk may very well not be a giant and Gaea may not be the biological mother of Thor, but because of the long-standing connection between Thor and Gaea and the fact that the Earth Mother claimed Hulk as one of her sons forces fans to follow a chain of logic that leads to the idea that the Hulk and Thor are brothers without solid confirmation or revisited explanation at any point in their subsequent stories.
This familial connection is truly a bizarre one and becomes even more confusing the more one lingers on the idea of it being true as it calls into question Hulk’s actual origins as well as Thor’s true parentage. However, at the end of the day, the validity of this revelation isn’t important as Hulk and Thor have been brothers since they first met in Avengers #1, and no amount of personal conflict or disagreement will ever change that, so the idea that they are literal brothers is irrelevant since they already created that bond between themselves years ago. Marvel made Thor and Hulk brothers by stretching logic to its limits, and even then it’s still questionable.