How to know if this relationship is a rebound?

I have been with a girl for 4 years 3 of which we have been engaged it wasn't working at all in the last year and we were fighting almost every day and for a month she wanted a break-up and I didn't but I finally saw that we weren't meant for each other and I broke up with her that was one month ago after that I didn't feel depressed or anything that I was expecting on the contrary I was relieved and I was starting to feel happy as if I already started moving on in the last period of our messed up relationship. My fiancé called me after a week and wanted us to be back together and she regretted her decision but I didn’t want that because we tried a lot before and we’ve given it a lot of chances and it never worked and neither of us is happy now and I don’t remember the last time we were happy. Ever since we broke up there is this one girl that was always there for me I've known her almost as long as my ex-fiancé because she was kind of related to her "their siblings were married" I've always liked that girl and we used to spend so much time together before and my fiancé was always jealous of her but now I'm starting to feel like I'm falling in love with her we have so much in common and we talk every day from the moment we wake up till we sleep. This is also very complicated because of her being related to my fiancé and I don't want to rush into a new relationship because it might be a rebound but what if this is real love?
She also broke up with her boyfriend 2 days after my break up.