What's the noun for littermate? Here's the word you're looking for.
litter
(countable) A platformmounted on two shafts, or a more elaborate construction, designed to be carried by two (or more) people to transport one (in luxury models sometimes more) third person(s) or (occasionally in the elaborate version) a cargo, such as a religious idol.
“The unfused interfrontal, coronal, and sagittal sutures are indicated in the skull of a wild-type littermate.”
“There is also a male tiger, Sasha, brother and littermate of Alexis, but as the only male, he is always let out alone.”
“On the other hand, the results gathered from non-stressed littermate show that they become easily exhausted and are near death if exposed to intense prolonged stress.”