Every family has one
I know it's probably difficult for your cousin to have to be accused of these things, but does anyone really believe the cousin who is making the accusations? I'd bet that everyone in the family already knows that this cousin has mental problems and has a history of rewriting history. From the story that she told about sneaking out of the house and then revealing the "rape", she sounds like the type who refuses to take responsibility for her own actions. Instead, she can remain blameless for all the failures in her life since she "had to do it" bases on the sins that she feels were committed against her.
Every family has one, and everyone in the family knows who it is. In our family, it's my husband's brother's step-daughter from his first marriage.
She has decided that I have dedicated my life to making her miserable, and has sent letters, e-mails, and made phone calls to let everyone know. I ruined her childhood, I kept her from doing the things she wanted as teenager, I kept her from going to college, I kept her from getting a good job, I keep her from, well, you name it, and it's my fault.
The funny thing is that I only married into the family 18 years ago and meet her just before the wedding. She was 24, so how I ruined her life is beyond understanding. She cussed out my MIL so MIL cut off contact with her 13 years ago. My husband and I hadn't spoken to her in 16 years since the day when she cussed me out when she started her campaign to let everyone know my sins.
Everyone let me know what she was saying, and they just shrugged and said "well, you know how Tanya is". Everyone let me know that they didn't believe her and that they think she's just a nut. Since no one believed her about me, she moved onto another target.
Like I said earlier, I'd bet your entire extended family knows not to believe this kook. If your cousin just goes on with his happy life, one of three things will happen: 1) She'll get help and learn to take responsibility for her own actions, 2) She'll find another person or event to blame for all her problems since she can't take responsibility for her own actions, or 3) She'll escalate her accusations to the point that he can take legal action against her.
Good luck, and remember: Living well is the best revenge.