How would you deal with your (14) teen having sex?

I’m a nurse and my husband is a contractor and he goes out of town sometimes not too often but he could be gone for 2-3 days I usually leave my 6 yo daughter 10 yo son and my 14 year old daughter with my 21 year old daughter (I pay her $300 a month) I had my (14) daughter on the pill for about 3 months and she kept forgetting to take it so we decided she should get the implant and she agreed. I can’t force her to use condoms but I do have the talk about stds with her all the time. I believe my 21 year old daughter is allowing her to go to boys houses and bring boys to my house when my husband and I are gone (I ask my 6 yo she said that “sissy had boys over”) emphasis on the “boy(S)” I hear my 14 yo and her friends ( I allow girl friends on the weekend) talking (I’m not using these boys real names) “are you still with jeff” “no he cheated on me but now I’m with caleb” “you should see his d*** it’s huge” and other things like that I usually stay out the conversation while her friends there but once they leave I talk with her (mainly so I can continue to eavesdrop) What I’m worried about is stds and her reputation I know in my teen years I was doing things I regret now and I wish everyday that I would have waited until marriage to give anyone that part of me. I want to be the parent that I needed to my daughter but I don’t know what else to do. Sorry if that was long.