My little sister saw me naked?

yesterday I was babysitting my little sister who is 11. I'm 13 and the oldest of us. I agreed to babysit her while my parents went out with friends. I told her to take a bath and head for bed. After she was done I decided to take a shower. I realized I left my pajamas on the coach. Thinking my sister was asleep I wrapped a towel and went to get them. My sister had them in her hands and wouldn't give them to me a chased her around until we got up stairs when my fowl fell. She droped my pajamas and started shouting out that I was naked and had a hairy private. Then she ran off to her room. I was so embaressed so I didn't say anything to my parents but today my sister started teasing me and saying that she's going to tell her friends. What do I tell her and should I tell my parents. She also showed me her underwear and started saying I have no hair down there