Baragund's date of birth does not occur in any canonical sources. The figure shown here is from the genealogical tables accompanying the Later Quenta Silmarillion in volume 11 of The History of Middle-earth.
While Baragund's elder sister does not appear in The Silmarillion, her existence is established in various sources (most notably Aldarion and Erendis, where she is described as an ancestor of Erendis of Númenor). The spelling of her name shows some variation, and she is also referred to as 'Beleth', but this genealogy follows the spelling used in Unfinished Tales.
In The Silmarillion, Christopher Tolkien's genealogical chart shows Belegund to the left of Baragund, suggesting that he was the elder brother. However, in all other sources (including many references in the text itself) Baragund is given primacy. It appears, then, that the Silmarillion chart is arranged for convenience of layout, rather than being intended to show the relative ages of the two brothers.