Home wrecker

Home wrecker, it it an actual thing? And if it is what determines if you are one? My sister today determined that I was one. Ever since I fully became friends with Sarah I have decided that I wanted to date her. I know it needs to be taken slow but at the same time there is someone else in the picture, Zach her boyfriend. They aren't having problems and he treats her like a princess, but she doesn't get butterflies or feelings. For me she does, more than all that. So the question then comes back to if I continue to develop these feelings and let her develop feelings with me am I considered a home wrecker? I am not for sure what I believe and the fact that we are still young I believe we have the feelings of he is my boyfriend one day and not the next. At the same time though that would make Sarah pick me today and not tomorrow, which I know will break my heart. So the idea comes, do I want Zach to be hurt and heartbroken the way that I don't. Let's now look at the person who I believe is most important to this puzzle, Sarah. Why should Sarah's happiness be compromised just because I came into her life during a time that she was with Zach. If she is happy with me shouldn't that be enough? In the same respect would she have broken up with Zach even if I wasn't in the picture? All of these questions lead back to one problem, at least on my mind, She hasn't broken up with Zach yet. So will she eventually do it? So let's go back to the beginning question, Will I be considered a home wrecker even if she doesn't break up with Zach? I don't know what to do really but just live it out and enjoy my time with her hopefully time will tell.