Bring sugar, 1 1/2 cups water, and juniper berries to a simmer in a pot, stirring until sugar is dissolved. Remove from heat; let cool completely. Strain through a fine-mesh sieve; discard solids. Refrigerate 1 hour or up to 1 week.

For each cocktail, combine 3 tablespoons juniper syrup with 1/2 cup sparkling wine in a glass. Garnish with a pine sprig; serve immediately.