Ysabel was part of the retinue sent to Dragonstone by Queen Regent Alyssa Velaryon, alongside novices Lyra and Edyth. She was sent to see to Alysanne's religious instruction and expected to impress upon Alysanne, and perhaps even Jaehaerys, that the Faith considered incest an abomination so the couple would then consent to the annullment of their marriage.
After King Jaehaerys I Targaryen wed his sister, Alysanne, Jaehaerys sent seven preachers to walk the realm and win the small folk over to the marriage, not with swords, but with words. Septa Ysabel was among these Seven Speakers. They traveled the realm alone, walking from village to village, telling the smallfolk of Jaehaerys's wisdom and Alysanne's kindness. When challenged on the issue of incest, they would spread the Doctrine of Exceptionalism.