This happened when I was in 2nd year in uni, and my sister had come to visit me from her school. She did not want our parents to know she was in town so she stayed with me. Just before bed she showered and came out topless! She then proceeded to ask me for a tshirt. Her nipples were so big, I found myself looking away but she kept it cool like it was nothing. Throughout that weekend she let me see them again, and it did a number on me. I found myself getting hard just thinking about them, lol.