How to make sure your little sister keeps something secret?

I am a girl btw. I live in a house with 5 sisters and 2 brothers, so things aren't that private. My youngest sister is 8. I have lately been a bit curious about sexuality and I asked my friend if I could kiss her to see what it felt like to kiss another girl. She was fine with it and let me do it. (I didn't like it btw) My little sister saw me doing it in my room and ran off, I had to catch her and ask her not to tell anyone. Unfortunately, she does have the annoying habit of blurting things out that she shouldn't even when she has been told. I really don't want my family to know about this because I am definately not a lesbian, I was just curious adnd I don't want to be labeled as one. How do I make sure she won't say anything to my brothers or sisters, short of taping her mouth and locking her in a cupboard?