This superstition is called “sukob” and advises against siblings marrying within the same year as it is said to divide the luck between the two marriages. Another type of sukob advises against marriages within the same year as the death of an immediate family member. Pushing to do so is considered bad luck.
Another wedding-related superstition is that the bride should never try on her dress before the big day. This is said to bring bad luck and cause the cancellation of the union.