It is a Chinese belief that siblings should not get married within the same year.
However, in terms of feng shui or Chinese Metaphysics, there is no such rule at all. In fact, whether be it Feng Shui, Bazi or even Date Selections, it is actually a form of Chinese Astrology. And astrology is about the positioning and configuration of stars, planets and constellations in the sky. It is both a science and an art. Nothing superstitious or religious about; and it is not a belief or culture.
Personally, I don't mix Feng Shui or any Chinese Metaphysics subjects with superstition, culture, beliefs or religion. They are separate distinct subjects. Even the ancient classical feng shui text written in the olden days of China does not mention anything about superstition, culture, beliefs or religion in it.
So to answer your question rainydae, personally I don't believe in it. There is no logic or basis in it.
However, since it is part of Chinese belief, and if you feel uncomfortable about having 2 siblings getting married in the same year, by all means avoid it. There is no harm doing so, and most of the elderly people such as our parents or grandparents believes in it. So if possible, i always advise doing it to make them happy