What should I do about this girl I love?

I have asked so many questions regarding this girl but I'm going to ask one more because things have actually taken a turn for the good! I came out of a messy relationship this time last year which had lasted a year, it was long distance (she lives 30 mins away). Towards the end of this relationship I started talking to the girl this question is about, and then a few months after we began talking as friends (flirting a bit because my relationship was sloppy) all hell broke loose in my relationship and then I began seeing this girl. We met a couple of times whilst I was still in the relationship at hers, but we just got to know each other at her house, nothing happened. My relationship ended whilst I was on holiday, the first time I met up with this new girl, we kissed. Then we continued to meet in the park after dark, and we'd talk for hours and hours and have a play around, it was really amazing. We've become incredibly close over the year, so close we are practically boyfriend and girlfried, but despite the fact we have even had sex a couple of times, she still tells she doesn't want a relationship with me. I agree to this because I think she is the most amazing girl on the planet, and I wanna keep her, no matter what our relation is, but everytime i see her we always kiss and do more, but still no relationship! So I am on holiday at the moment, but when i'm back i've been planning to ask her to be my girlfriend, we love each other so what do i have to lose? Her family adore me, she adores me, i may aswell. But then yesterday she told me that she wants to have sex again when i'm back (we haven't in 3 months), is this a good sign and a plus indicator for me to ask? If so, when should i pop the question? Before sex, after, the night we do it, i'm not sure!! Thanks you lovely people!