To get Isabella, just go into The Hanged Man bar. I'm not sure what exactly triggers her event in there, but if you've recruited everyone else you should be golden. I have no idea about Sebastian, since I don't have that DLC.
Guild Wars 2, Crystal Desert server:
Inga Freyasdottir, lvl 40 Norn Mesmer
Deathbringer Kaami, lvl 16 Asura Necromancer
Popina, lvl 15 Asura Elementalist
Althaia Willowbark, lvl 7 Sylvari Engineer
Maeve Amnell, lvl 4 Human Guardian The MMO-Zam's FB group.
Please message me first so I know who you are.