Outside this canon many of us like to assume they’re related because they’re all Canadian. It’s the law of Minecraft National Kinship: People on a Minecraft Server from the same country are related in some form or another.
People assume that Etho, Gem and Beef are siblings because no matter how they change their appearance they always still have elongated elven like ears.
They are not actually siblings, they just come from one civilisation that came from colder climates to warmer ones and bigger ears (just like with fennec foxes) are basically an evolution that helps them not get too worm in that warmer climate.
And also the reason why they can layer clothes and not feel too warm in any biome, they already are ventilating !!!