Should I date my best friend?

Ight, so I know I posted this earlier but I forgot to add a very important fact. So here is how it went down. I have been friends with this girl for awhile now. We are best friends, she is like a sister to me and I'm like a brother to her. We told each other this. However, recently she told me she told me tonight that she has been developing more feelings for me. Here is what happened alright. She told me she is sorry for putting me through a lot of stress because I at one time had feelings for her but she didn't feel the same. She told me that she was stupid and didn't know what she wanted in life. She said that she thinks she always loved me but didn't realize it and that I am the only guy that understands her and has never judged her. She said that she has never been this close to anyone before(she doesn't have many friends) and that she was afraid in the past. She ended the conversation telling me to think about if I want more with her or not and that she was really sorry for breaking my heart in the past by telling me she wasn't interested. What should I do? Is it really possible to go from a best friend to a boyfriend when the girl has said she didn't see you that way in the past? Am I setting up for a failure or should I give it a try and finally date and possibly spent the rest of my life with my best friend?