Sisters had sex at age 13?

Hey there,

Well about a month or two ago me and my 13 year old sister were watching a movie. When we watch films she always gets into my bed with just her panties on and I always just have my panties on too, it came to a seen in the film where two girls where kissing then for some reason my sister rolled over to face me. After she rolled over her panties hit my hand and I could feel they were wet. When she was facing me she started kissing me and I enjoyed it. But the strange thing is my vagina started tingling and I enjoyed it. Soon after her kissing me she slipped her hand down into my panties and she started running her fingers up and down me. Soon I felt her fingers go up inside me, it felt very good so I put my hand down her panties and did the same. Soon we were both fingering each other then my sister brought her hand out from my panties and started licking the juice-type liquid from her fingers. Both our panties were very wet and we started rubbing them against each other. Soon after she had slowly moved her head down under the sheets and I felt her tongue on my panties, she soon slid my panties down and I could feel her tongue moving up and down my vagina. I started screaming because it felt that good then I started fingering her at the same time then she started screaming too. We both enjoyed it and now do it ever weekend almost. Is this ok, is this normal?