I'm in love with my friends girlfriend, what do I do now?!

As the title says, I'm (M25) falling in love with my friend's girlfriend (F30ish), they are together 3 years or so.
It's not a crush. I'm deeply in love with her, she has a daughter from a previous marriage and I have quickly come to care for her too. She and I get on great. I find when I am not around them I miss them both so much. I didn't mean to fall in love with her, I didn't set out to do it, but here I am writing this post. You can't help who you fall in love with I guess.
I don't know for sure if she feels the same way, but I know we are close. She confides in me a lot of the time. She is flirty and she seems happy when we are both together.
They are on and off the rocks constantly. He has anger issues and is a little controlling, he's pushing her away.
Her daughter and I get on great, as strange as it sounds I'd treat her like my own. She has even said it several times to me with her Mom present " I wish you where my Dad " my friend is not a kid person, don't get me wrong he's nice to her, but not in the same way, if that makes sense, i just get her the time she needs.
My friend and I get on great, he has a lot of good qualities and there is one part of me that's saying " don't do it, for his sake " but it's his bad side that is edging me closer and closer to acting on my feelings and telling her how I feel.
Ugh what do I do ?! Suck it up and be un happy ? Stop seeing them, which would destroy me.. I don't want to be a home wrecker either.
TL;DR: have fallen deeply in love with friends girlfriend, and care a lot for her daughter. Don't want to ruin anything but it's tearing me apart not being able to express how I feel. What do?!