I'm having a party with Ifan and Sebille, considering what I've read from other's comments/inputs; I teleported Sebille at the Driftwood-Square so that Ifan can talk to Roost without Sebille intefering. Interestingly, it doesn't matter whether if Sebille will confront with Roost or not because after killing Roost, Sebille's personal quest stated: Roost Alon, who delivered Sebille to the Master, is dead. The Master is next and her log doesn't closes and as for Ifan: You just know that Alexander is still alive and Roost didn't reveal the identify of the one who issued the Godwoken contract. In short you can proceed with the story even if Sebille is the one who confronts with Roost. Cheers.