What hints are there about George and Lennie about their relationship?

The relationship between George and Lennie begins when they are both young and Lennie's aunt, Clara, dies. George promises her he will take care of Lennie, a man who is cognitively delayed. By taking care of Lennie, George essentially gives up his personal freedom as well as his job security; Lennie will prove to be problematic everywhere they go due to his uncontrollable fits, his use of extreme strength, and his imposing build. 

Still, their relationship is symbiotic; both men need something from one another the very things that sometimes come as obstacles in their relationship. For instance, from the start of the novel, we learn that George, a relatively small-sized man, is in control of the duo. George calls the shots, decides where and when they will stop for the night, and he scolds Lennie when the latter acts out his strange behaviors. In the first part of the story, we see how George calls out Lennie about the mouse that Lennie is carrying in his pocket, which he accidentally kills from petting it too hard. George consistently accuses Lennie of being slow, of ruining his (George's) life, and for getting them in trouble all the time; all of these accusations, however cruel, are also true. 

Still, they remain together. Why? Because Lennie's strength and size compensates for George's smaller, less impressive frame. On the other hand, Lennie depends almost entirely on George's wit and quick thinking because Lennie's own mental capacity is limited. As such, he is the body while George is the brains. Together, they form one whole person. This is where the symbiotic nature of the relationship is evident. 

Moreover, they also remain together because they have absolutely nobody else to rely on. They have no family, no other friends and, as such, they greatly benefit from being each other's protectors and supporters.

We got a future. We got somebody to talk to that gives a damn about us [one another]. We don't have to sit in no bar room blowin' in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us."

On top of it, they do share a dream together: the dream of living "off the fat of the land" and being able to enjoy the benefits of farming for themselves, rather than doing it for others. These variables keep the men together. All of this gives us hints that their relationship is best considered fraternal because they act as if they are brothers that watch out for one another (probably better than brothers do), and they are also interconnected at many other levels: for protection, for support, for friendship, for company, and to help one another keep the dream of the land alive.