Hello. Thanks for writing to us at healthcaremagic Unprotected sex if it is exclusively between the two of you poses no risk of any STD, whatsoever. However, if either one of you is or was engaged in sexual activity outside of it with someone of unknown sexual status, then there is a chance of either of you to get infected with STD s Regarding the sexual encounter when she was 12, even though that was a long time back, I would still recommend that both of you get tested for STD s including HIV, Hepatitis B virus (HBV), Herpes Simplex Virus (HSV), Syphilis (VDRL/RPR test) because some STD s can remain clinically silent for years (like syphilis, herpes, HIV etc) and because you cannot be sure about the sexual status of her brother, whether he was carrying an STD at that time. My guess is that she never developed any symptoms or signs of STD s like genital sore, growth or wart, lump etc during all those years. Regards Always practice safe sex with a condom!