First I want to share with you that I am really sorry to hear that you feel your parents treat your sister better than you. In my personal experience that would make me feel sad, and question what it is about me that makes them treat my sister better...that being shared, would you consider taking stock of this situation, and maybe try to get some perspective, to try to understand if your parents are truly treating your sister better than yo, or if this is perhaps, just perhaps, a perception that you have that isn't actually the case? I would never invalidate how you feel, I just think that, in my personal experience, it is very easy for most of us, as human begins, to get focused on what we percieve as being slighted...another option you could consider is working with a mental health professional. You might also consider focusing on taking grew care of yourself, and spending as much time with friends and other family that support you. I know you asked why your parents treat your sister bette than you, and I will share with you that. personally, in my experience, if the truly do treat your sister better, you may not ever get to know that answer to that question, and that would require a great deal of acceptance on your part, if you are willing to go down that path in your journey with this. However, you could, if you feel comfortable with it, ask your parents to have a heart to heart with you about your feelings on this. Or you might consider writing a letter to your parents if you feel more comfortable with that. You know you and your parents and your feelings and the relationship you have with your parents best, so of course these choices are best made by you. These are just ideas I wanted to share with you from both my personal experience and what I have observed in supporting other friends and families with situations that I perceive as similar to what you briefly asked here.