Hi, I'd like to help you with your problem.
This is a difficult yet common situation that people find themselves in when they have a relationship with someone who has children. I understand from your question that it is your boyfriend's eldest daughter who is manipulating the situation and showing you her dislike of you when her father is not around. She is also involving other younger children.
Dealing with this issue is never easy. However, there are things you can do to help improve it. First, you need to sit down with your boyfriend and explain what is going on. You may have already done this, but try it again. Set aside a time when you can be alone with him and no one can overhear or interrupt. Explain in a non accusatory way what has been going on. Let him know that you want to improve your relationship with his daughter and ask how he can help you achieve this. You also need to decide together the boundaries and rules for his daughter. He must agree to this and back you up if she resists.
Also, talking with your boyfriend's daughter is another option. It will not be easy, but it's worth a try. Let her know that you are not there to replace her mother and that you just would like to be friends. She may have a lot of anger about her parents relationship and also feel you are a threat to her "daddy's little girl" status. Reassure her that you will not come between her relationship with her father and that understand it is special. However, you are part of her father's life and if you are willing to respect her relationship, she needs to give you the same courtesy.
The main point here is to provide a united front with your boyfriend. She will never respect you as long as her father allows her behavior. If you cannot get your boyfriend to work with you, suggest short term counseling to help you both work this out.
I hope this has helped,