Who is Rosaline in "Romeo and Juliet"? A Capulet? A singer?
Rosaline isn't a singer! She's the girl Romeo is in love with as "Romeo and Juliet" opens, who never speaks any lines, and who he forgets more or less as soon as he sees Juliet. It's a pretty laughable intoxication, and, though he certainly talks in a frustrated-lover's language, it's difficult to believe that his affection for Rosaline is anything to compare with what he later feels for Juliet.
You are right though, to assert that Rosaline must be a Capulet. In Old Capulet's guest list, as read out by his servant Peter, you will find among those invited 'Mine uncle Capulet, his wife and daughters, / My fair niece Rosaline, and Livia'. So Romeo's first passion is also for a member of the Capulet tribe: which, I think, rather undercuts any notion of the "forbiddenness" of a marriage between a Capulet and a Montague.