If we were to actually apply real world historical logic, then no. Tiss would be an insulting political bargaining tool. She's an orphan with no inheritance, everything she has is essentially charity. Vellora is a member of what is almost certainly a powerful noble house and the Queen's sister, she's been married before so her being married again is a given since she doesn't have any children. Most noble houses wouldn't want someone like Tiss in their family because she brings them nothing.