My Dad Has Another Daughter

Hey, so I'm 25, and I just found out the other day that my dad has another child, so I have another sibling! Now I always knew that my parents were on and off, boyfried and girlfriend, who happened to also decide to play house my whole life. But did not know I had another sibling via my dad and another woman until this past Friday, when my cousin happened to see me at my job and mentioned that I looked like my sister. Apparently, my dad showed him a picture of a girl who looked like me and told him that was my sister. And I was mad, confused, and told him, "I don't have a sister", just a brother, who is my half-brother via my mom and her ex. But obviously, my cousin didn't know that I wasn't informed that I had a sister. So he just continued to tell me that she went to our high school. Although I was confused, I just moved on, but yesterday I decided to tell my mom, and she confirmed it, but never wanted to tell me since it was my father's matter, stating that while she was pregnant with me and living with my father, his "girl" at the time came to pick him up from their appartment and she happed to be also pregnant. But he has only raised me as his only daughter for the past 25 years. So knowing all this, I just need help addressing this. How should I tell my dad that I know about his other daughter?