Who should I Tell*****?

Ok...I have a 13 year old sister, shes been with a guy for about 6 months now and she just told me yesterday that she had ..(she told me about 4 days or so after she actully did it...).im really worried because i know how guilty you feel after the first time...i was 14 when i did, and it was the biggest mistake of my life...besides that..i dont know who to tell..im really close with her and i dont want to make her think i "tattled" on her..help please?