My sisters best friend has a boyfriend and I love her so much but I have already asked her out but she said no and I want to go out with her so bad but she loves her boyfriend and he loves her
Please help

Check out this thread for guidelines:
https://www.askmehelpdesk.com/dating/guidlines-what-do-do-if-person-like-already-relationship-463250.html

You already made your move and she already gave you an answer. So now it's about accepting her decision. We have no idea whether she will change her mind about you, but for now, she doesn't want anything with you.

You can keep trying to get to know her if she's willing to let you, but I wouldn't try pushing your feelings onto her anymore. You're better off just as friends for now, especially since she already knows how you feel and still has a boyfriend.

You may be waiting for something that will happen, so be open to meeting new people, because you shouldn't have to put your life on hold.