Trying to understand a new relative

Hello Reddit. I am a young guy who is trying to reach out to a new family member. The information is kind of jumbled together, so I'm not sure what to make of it. I was born without any idea who my father was or what became of him. When I turned 16, my mother contacted my paternal aunt, who sent me the documents from a trial related to my father's passing. It wasn't until I turned 18 that my mother, along with my two sisters, went to visit the aunt and two cousins, the latter of which are my father's children. If you're still following, then you know that we have an aunt who is my mom, uncle who is my father as well as the mother of my siblings, and my cousins who are my siblings. Upon seeing the documents, I now understand that my siblings are my half-sister and aunt. You see, they are the children who were given the death sentence, and it is because of this that I am now reaching out to them. My sisters are very close to me, and I recently found that they are also curious about my half-sister. It's for that reason that they inform me to reach out to them, so I did. They seem very friendly, and they tell me that they enjoy writing letters to pen pals. The truth is that I am not really sure how to relate to her. She is my aunt, sister, and cousin. What should I be talking about? What are the dynamics of this relationship?