Why are people shipping Ramyatta? Aren't they brothers?
The short answer: no. For the long answer, read on.
“Zenyatta and Ramattra call each other 'brother.'”
Zen and Ram are both former members of a monastic order. The Shambali call each other “brother” or “sister” as a sign of respect, endearment and unity within their religious community, just as they do within their monastic, real world counterparts.
How do I know this? Because Zen also calls several other characters his “brother” and “sister.”
On the Nepal map, Zen has a voiceline stating “I have not returned here for many years. I wonder if my brothers and sisters will be pleased to see me,” referring to the other Shambali.
Zen also shares a voiceline with Orisa when he uses the orb of harmony on her: “We are as one, my sister.” In OW2, he has voicelines with the robot on Push Missions: "Hello, brother. Come with us." And when the robot is taken by the enemy team: "The enemy has stolen away our friend."
Generally speaking, the word brother has more than one meaning. English has no word for a man to use in reference to another man who is more than a friend but not a lover. Brother has been used as stand-in in this context forever.
“The devs said they were brothers.”
The tweets and videos so far released by Blizzard have mentioned that Ram and Zen are “as close as” brothers. They have also referred to their relationship as a friendship.
“What about Mondatta?”
Mondatta is also not Zen’s brother (other than in a monastic sense, see above). The "name" Tekhartha is actually a title or honorific. This was confirmed in a 2018 forum post by former lead writer Michael Chu.
“What about adopted/found family?”
Zen and Ram are robots. They were not born or raised together. When they met, they were functionally adults both making their ways in the world. This isn’t considered incest by any stretch of the imagination.
“I just prefer to view them as platonic/brothers, ok?”
You may of course have those HCs. I only ask that the same courtesy be extended to fans who prefer to view their relationship as romantic.