Venus is often called Earth's “sister planet” because the two worlds are remarkably similar in size and mass. Venus is about 95% as wide as Earth and has roughly 82% of Earth's mass, giving it a comparable rocky composition and surface gravity.
The resemblance ends in dramatic ways. Venus has a dense carbon-dioxide atmosphere, sulfuric-acid clouds, crushing pressure, and a runaway greenhouse effect that makes its surface hotter than Mercury's, despite Venus orbiting farther from the Sun. Earth, by contrast, has liquid surface water and a nitrogen-rich atmosphere.
Venus is also the closest planet to Earth in size, but “sister planet” does not mean it is Earth-like in climate or habitability. Its slow rotation is unusual: one Venusian rotation takes about 243 Earth days, longer than its 225-day trip around the Sun. Venus rotates backward compared with most planets, so the Sun would rise in the west.