Although it is a difficult position for you to be in the fact that your cousin trusts you is a very good thing. I would let her know how much you appreciate her trust in you, and how much she means to you. I would then let her know that you are concerned about her medically. That you will go with her to see a physician. This may be the easiest way for her to get some of the help she needs. A physician will see the marks and know that she will need some help. Show him or her your marks as well. You are a good cousin and helping her — and you — get some medical attention is the first place to start.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral