is it a sin to love someone already in a broken relationship?

I love a girl whom I've know for over 6months.And she loves me too.But the things is she is in a relationship with a another person for more than 4years.currently they live in different states.She confessed to me that the there doesn't exists any more love and trust between them.That's when i came in and took care of her,helped her was with her whenever she was in any kind of tension.We both fell in love.And decided that we should marry.All this was going on without her boyfriend's knowledge.
As everything comes to light at some point,her boyfriend found out about us as he used to keep tabs on her emails(how cheap can that be) and they had arguments for days.At first she said I'm scared and hope he doesn't hurt me,cause he had come down for a month to meet her.After a few days passed she said I'm totally confused and need some time to figure out stuff.Later on she tells me, we can only be friends,i cheated on my boyfriend and i can never forgive myself for doing wrong to him.
She used to care for me so much and know she doesn't even bother to call me,meet me.I'm deeply in love with her.shes the most beautiful thing that has ever happened in my life,she always brings out the best in me.I need her each and everyday of my life and I need to get her back at any cost. I have to.
I need some help and advice!!!!plzzzzz

Yeah it's a sin and your going to hell.

I don't know if it's a sin, but I think it's a bad idea to get involved with someone who isn't available. That's the recipe for heartbreak.

A relationship as bad as theirs shouldn't last. If she ends it, which I hope she does, she knows where to find you. Until then, don't get too distraught about it. Try to find other things to think about.