What Sinon knows prior to the revelation:
- guy likes her
- guy cares for her, is her only friend
- instead of clearly telling him off, she gives him a reaction causing him to say "okay, i will wait", to which she doesn´t object
- she is fine with going on like that
So yes, she IS the bitch in this scenario. Why? Because she´s instilling hope in this guy, who SHE knows that he wants more. As Kraco mentioned, sure, she has this trauma. But that doesnt make her less a bitch. It simply might explain why she is a bitch. And just for fun , Ill write the word bitch some more. lol.