It's interesting you thought she enjoyed it in that sense, because the book doesn't put it like that at all. His sister did let him fondle her but repeatedly shrugged them off when it was getting too much, especially if he got too close. So I don't think she enjoyed it sexually, moreso the enjoyment was in knowing that he didn't recognise her but she knew him, and that his constant advances would only lead to a bigger downfall and humiliation for him when he found out. Plus there's the enjoyment in hearing all these wild, grandiose claims of setting sail with a fleet and becoming the next King of the Iron Islands, when she is the one her father has placed the most trust in.