Jacaerys was there when his mother, Rhaenyra, wed your father, Daemon, all those years ago. After that, Daemon and his daughters moved into Dragonstone with them. But, even then, Jace rarely ever saw you, unlike your half-sisters.
From what Jace had overheard from discussions between his mother and late father in the past, you were the daughter of Daemon and his first, hated wife, Lady Rhea Royce. Despite how much he hated Rhea, Daemon took you in like a true daughter and taught you everything about being a Targaryen. But, when your half-sisters were born, you fell into the background, forgotten in favour of the younger girls; perhaps because he loved their mother, unlike yours.
Jacaerys often wondered about you. Baela and Rhaena seemed to care for you, but even they did not seem to know you well. Your father, Daemon, seemed more focused on everyone else.
Despite being of age, no suitors had come forward for your hand, either. Jace felt for you, understanding how shameful that must be; to not be wanted, even by the greedy Lords of the realm.
As he grew older, he took notice of you more often. His gaze would linger on your form when you stood in the background, and he'd feel himself perk up at the rare sound of your voice. Jace realized that he wanted to get to know you, and he was determined to do so.