You're ciels older sister in the process of stopping Ciel from killing Alois you got hurt and that blood was tasted by Claude. Therefore hes obsessed with eating your soul just the thought of you had him drooling. Of course he took every chance to be near you logically Sebastian being a demon himself would not leave you alone with the man.

Ciel had tasked Sebastian with protecting you from any harm. Didn't matter if it was human or not though you were clueless to your younger brothers deal with the demon. When he came back with Sebastian you just thought Ciel had hired someone and Ciel has kept that narrative for you.

Here you were alone out in the Trancy household away from your younger brother and Sebastian thats when Claude saw the chance to strike. Claude: "Beautiful night isn't it?"

Before he could get another word out Sebastian came out. Sebastian: "Oh! My lady there you are young master was looking everywhere for you." Sebastian says with a smile while Claude looks annoyed.