That's a tough worry to have on your heart long term. You want to be supportive, but will anything you do ever be enough? Possibly not. I would stop getting the books for her, as my first step. Secondly, I would try and spend regular hunks of quality time with her. Thirdly, I would consider my boundaries in the relationship with her.

You're not responsible for giving her a reason to live. Each person must work that out on their own. Your responsibility is to love her and be available to her in times of need. That does not mean she gets to pre-empt your plans for the day on a regular basis. When someone attempts suicide repeatedly, you have to consider that these attempts are meant to have an impact on others. She wants to change how her bf treats her, so this could be an attempt to emotionally blackmail him. (If I die, it will be your fault.) She's not going to change him any faster than you are going to change her. She possibly hopes that you and your other sister and your mom will put pressure on the bf to improve the relationship - make him feel guilty. That's not likely to happen either. She knows what she wants and is desperate to get it. Sometimes, in life, we just can't have what we want, which can be hard to accept. Sometimes we have to come up with Plan B. Your anger is understandable. You may feel like you are being manipulated, which I think you are.

You must free yourself from any notion that, if your sister does manage to end her own life, it will be because you failed her. That's what I mean about boundaries. You have your own responsibilities to take care of. Schedule quality time with your sis, but beware of being overly available. She has responsibilities too. Maybe ask her to do you some favors and to help you out with this or that. Depressed people are inclined to become self-absorbed. (I know that first-hand.) Don't reinforce self-harm behavior by overly rewarding it with excessive attention.

Your sister is in pain. That deserves empathy. Avoid comparing your sister to yourself. I can understand anyone wanting to leave this "vale of tears." It can seem like an effective way to end pain. As we mature, we come to understand that some pain goes with being alive. Your sister is having trouble growing up. Remind her of the challenges she has already met in life. Express your faith in her. This may be the moral support that her bf fails to provide. Let her express how he lets her down. Affirm what can be affirmed.