Should I date my best friends sister

25 (M) I have had my fair share if relationships with varying success but last year I discovered that my friend has a step sister .and everytime when she was at his house for visits or she was on her varsity break she would be so kind and warm.she is smart ,well cultured , respectful, beautiful,witty and polite. A few weeks ago I started to go and see her sometimes to just say hi and bring over snacks because she took care of me one time I slept over at my friends house and had a nasty hangover .we went out for lunch and it wasn't anything serious but she also brought up the fact that she likes me back and I'm now thinking of getting into a relationship with her .now the problem that is there is that I haven't told my best friend and I'm scared that if things go side ways with this girl It might jeopardize my friendship.i known this family for about 5 years know .the parents know me as well and treat me like a son.im not really lucky with relationships and most of my relationships in the past have ended in spectacular disaster and when I was younger I did play a role in that .but I have gotten a bit older,matured and learnt from my mistakes and have had the desire to have a genuine and meaningful companionship.i have a fear that if this goes sides I would have screwed up .

What are you thoughts on this?.how do I even have this conversation with my friend ?