Sin-Sin is a weird champion with different roles at different stages of a game.
Early game she can be a tempo play that allows you to get ahead in stats on the table because of her “delayed” cost.
Mid game she is decent damage absorption. If you time her right you can soak damage with her and then move her out of harm's way, hopefully by reaching the opponent's backline to use Infiltrate.
Late game she can be a value play, as when played late your opponent may have no use for the magic from Bounty even if they kill her, thus she becomes a very efficient 2/6 for 0 cost!
At every stage you have to check if the conditions are right to fulfill her mentioned roles, otherwise you should readily discard her.