How can I help my sister?

My sister and I have always been very close, we share a lot with each other. But I've grown increasingly concerned for her as she's had over 40 sex partners that I'm aware of and I know she's not being "careful". She's 18 and I worry she is getting a name for herself - let alone the fact she could get pregnant or catch an STI. I want her to change but I worry if I tell her to stop being so promiscuous she will do the opposite. I know it is her life and her choices but I believe she is making mistakes and men will not respect her.