Keith and Shiro are actually brothers theory
So theres quite a few who also believe/think Shiro and Keith are/could be brothers. Well I have a theory (like i said theory) that they actually are brothers.
1) Galra tech. Shiro has an arm that we all know is made by the Galra. He has the ability to 'awaken' the arm and bring out its powers to fight the Galra who also have an arm like him. The thing is, no ordinary human can actually do that unless theyre part Galra. So all in all, its like the blade of Malmora;
2) Eyes, facial structure: although Shiro's jawline is more squared and Keith is more sharp, the duo deem to hold the same facial structure as the other. They hold soft cheeks, strong necks and jaws, and their eyes are the same color. It might be that they just have the same eye color like people who have blue eyes, but they dont have the exact same /shade/ and coloring unless theyre blood related.
3) Shiro and Keith knew of each other personally.
Now they could have been in the same class but that wouldve been a little coincidental. Keith spoke to him as more of a sibling instead of a worried friend. Keith and Shiro dont share their backstory, but it sounds to me that they go back farther than the garrison. Theyre close- but not like Lance and Hunk close.
4) Keith being nominated as leader;
look, if you were leader and you had to nominate a heir/second in command, you wouldnt hand it off to the most hot headed person of the entire galaxy. I like Keith, and he can be a good leader except he leads more with his emotions than his level headedness. Shiro makes a comment about this when he was wounded and bleeding out on the desert like planet they had crashed on in S2. It could be just a friendship thing, but leadership usually runs in a bloodline. I have a feeling that this leadership skill/quality runs through Shiro and Keith because of siblings.
5) The creators of dreamwork make everything possible to make it clear that they wont reveal Shiro's and Keith's past;
Remember in S2 where Keith was in that dream state and he saw his dad? Well if you look back over his father had a jaw much like Shiro's and that amazing 'eyeliner' of lashes. Coincidence? Possibly. Same eye colors too, and the same broad shoulders. So why does Dreamwork hide the Galra mother? Because its most likely that the Galra mother looks like Keith. Streamlined, and sharper, angular features than her lover.