Rosaline is Capulet's niece, as stated in his guest list, which he gave to his illiterate servant and which Romeo read:
"Signior Martino and his wife and daughters; County Anselme and his beauteous sisters; the lady widow of Vitruvio; Signior Placentio and his lovely nieces; Mercutio and his brother Valentine; mine uncle Capulet, his wife, and daughters; my fair niece Rosaline; Livia; Signior Valentio and his cousin Tybalt; Lucio and the lively Helena." (From Act I Scene ii)
However, it still raises the question: why would Romeo be dating a Capulet and not keep it secret as he later does with Juliet? Because of the family feud, wouldn't it be just as bad for Romeo to be courting his enemy's niece as his daughter?
At first, he is love sick with Rosaline, his neice, but when he attended the Capulet's ball, he instantly forgets about Rosaline and is deeply in love with Juliet, a Capulet.
he learns that there is a party at the Capulet house. he learns Rosaline is coming and romeo wants to see her
Rosaline was on the guestlist. If she hadn't been, Romeo wouldn't have bothered showing up. Rosaline was invited because she is a Capulet, Juliet's cousin.
At the beginning of the play, Romeo believes he loves Rosaline, who is determined to become a nun. In fact, Romeo attends the Capulet party in hopes of seeing Rosaline (because she is a Capulet), but instead he meets Juliet, who he also believes he loves (and perhaps he actually does).
They see a copy of the guest list.
At first, he is love sick with Rosaline, his neice, but when he attended the Capulet's ball, he instantly forgets about Rosaline and is deeply in love with Juliet, a Capulet.
he learns that there is a party at the Capulet house. he learns Rosaline is coming and romeo wants to see her
Rosaline is Juliet's cousin and Romeo's crush before he sees Juliet. He had a thing for Capulet women.
She got invited to Capulet's party, which made Romeo want to crash it.
Rosaline was on the guestlist. If she hadn't been, Romeo wouldn't have bothered showing up. Rosaline was invited because she is a Capulet, Juliet's cousin.
At the beginning of the play, Romeo believes he loves Rosaline, who is determined to become a nun. In fact, Romeo attends the Capulet party in hopes of seeing Rosaline (because she is a Capulet), but instead he meets Juliet, who he also believes he loves (and perhaps he actually does).
Rosaline Capulet.
They see a copy of the guest list.
Juliet, of course! The reason he went there was to see Rosaline but as soon as he lays eyes on Juliet he's crazy about her and completely forgets Rosaline.
Well, it depends on what you think the "first Capulet" was. The first Capulet Romeo falls in love with was Rosaline Capulet, Juliet's first cousin. Juliet was, of course, the second.
Roseline never actually appears on stage in Romeo and Juliet. She is the woman who Romeo is infatuated with at the beginning of the play, and, in a way, she is the reason Romeo and Juliet meet. Romeo goes in disguise to the Capulet's party in hopes of seeing Roseline, but instead sees the beautiful Juliet. He immediately falls in love with her, and Roseline is forgotten. Although Roseline is not described in detail, we do know that she is said to be extremely beautiful, and has taken a vow a chastity till marriage, (and therefore will not return Romeo's affections.)
Romeo's infatuation with Rosaline causes him to crash the Capulet's feast where he first encounters Juliet. That's the best answer I can make from the vague question.