Is it normal when i don't show interest in a guy, he goes after my sister?

I'm pretty close with my sister, she's like my best friend so it's common that we share the same friends. Generally guys tend to go for me more, because I look like I'm an easy lay. They then learn that I'm not the type of person to just go for anyone and then move on to my sister lol. Theres been instances that she wouldnt take them cuz they arent goodlooking enough for her, then they try going back to me. Is it normal for a guy to do that, when its blatantly obvious that it's a turn off when guys do that? Why bother keep going for one of us, if they keep failing? And another note, my sister and I are complete opposites...