What I am writing may not be a popular answer, but it is an honest one, based in reality.
Only you know your mother and family. Therefore, only you know if it is worth the risk to open-up to them. Not all parents are supportive, some blame their innocent child and further add to the trauma.
That said, it is important that you get help from some one, some where, and the sooner the better. You are a very strong, and resilient 14-year-old. Brainstorm which adult has in the past been the most accepting of you. If your parents don't regularly take your thoughts, feelings, and needs into consideration, then I suggest side-stepping them.
Is there a local rape crisis helpline you can reach-out to? Explain your concerns, especially if you know your parents will not be supportive and nurturing.
I will pray for you to get all the help you need.
Please know none of what your cousin did was acceptable. Yes, you were sexually assaulted. No, you are not to blame. You were a victim, not the one responsible. You are now seeking help, that means you are no longer a victim. You will emerge a victor, and perhaps you'll have to be the lone hero in your own life. But if you seek carefully and diligently, I know you can find someone to help you navigate this crappy situation.
I will keep you in my prayers. May safety and care find you.