In the theories of origin of the Moon, the sister hypothesis is so called because it was thought that the Moon formed as a separate body, at the same time as the Earth, from the same matter as the Earth - as if they were siblings, born of the same parents. It's quite common to use the word "sister" to denote related things, e.g. "sister ship" - a ship made to look similar to another ship, and sailing in the same fleet.