I’ve gotten myself WAY in over my head?

Ok so my best friend is 13 (i’m 15) and she’s dating this guy who’s 16. Bryan(her bf) and I have been talking since about thanksgiving and i’ve gotten to know him pretty well. He calls me his little sister and I call him my big brother. Bryan is currently in rehab for a Crystal Meth addiction. He and I talk almost everyday and he’s getting better slowly, but I talked to his younger brother(long lost, they just met) and he said that Bryan talks about me constantly and tht he thinks Bryan is in love with me. I’ve done as much as I can to help Bryan fight his addiction and I love him like a brother. I really don’t want to hurt Lindsey, but I think I’m starting to love him as more than a brother. Help!!!!
He’s not my actual brother….my best friend is like my sister so i considered him my brother….but getting to know him i’ve realized he’s more than a friend to me.