Canon Background
https://awoiaf.westeros.org/index.php/Sylva_Santagar

In-Campaign
Arianne and Sylva were introduced to Summer by Thomas in the final days of 286 AC. The children had met playing a game called Prince’s Pass on the beach beside the disabled Rambis within sight of Spottswood. There Sylva bragged of her father’s sand steeds, piquing Summer’s interest.

Arianne and Sylva stowed away on the Rambis during its sea trial, following the repairs to its hull made by Othello. They were discovered their by Lord Teddy Jasper. During questioning both girls revealed an interest in adventure. After hearing that Teddy enlisted their help in rescuing Ser Dolins and their friend Thomas from captivity at the hand of the Ironborn. Othello witnessed the agreement but took no part as it was not his place to speak amongst lords, ladies, and princesses.