He probably just meant that you're likely to attract attention from all kinds of people, including attention from people who won't be good for you. I think he just cares a lot and obviously loves you to death, so he wants you to use your very best judgement when it comes to the people you choose to spend time with, and to not be naive.
I think you turned him on. I think he walked in, your beauty caught him off guard and he said something without thinking about it and realized he might have accidentally said something bad.
It is not uncommon for boys to be attracted to their sister. I think he had a sexual thought about you and now feels like he is a perv and wants to distance himself from you to prevent that thought from re-occurring.
I bet one of his buddies said something about you. A lot of my friends who have big brothers have slept with most of his friends because they have "easy access" to hanging out with her and she often trusts them because she's grown up knowing them. Not all of them have honorable intentions, even if they seem like your own brother + attraction. A guy won't want to rat out his buddy (or bring up his sister's sex life) but he will feel protective of you and not want you to get taken advantage of.
He is protective over you and does not want you to be hurt ever,and that's good.Just be smart about everything,and do not use your body in the wrong way,then ull be fine.I mean RESPECT YOUR BODY and do not let anyone to love you only just cos of your looks...