When you are 40, will you still limit your dating pool because a 35 year old is the same age as your sister?
I’ve said this before. When I was 26, a hot 22 year old was after me. I blew her off because I thought she was too young. When I was 42 and recently divorced, a hot 22 year old was after me. Never let it be said that I don’t learn from my mistakes.
I see where Soapbox Monkey is perhaps coming from.
His whole teenage & young adult life there has been this *child * around him, his little sister. He’s finding someone in that ‘child’ age group attractive.
And, it’s not just that 18 is now associated with a child in his head, but his little baby sister could be screwing his classmates!
Let’s examine that pesky Age/2 + 7 rule referenced in the xkcd strip:
If she turned 18 six weeks ago, she would be about 18.115 right now. I’m just going to say you are 23.0 for simplicity. Now, for dateability,
Girl’s age > Your age/2 + 7
But GA = YA - 4.885, so we now have
YA - 4.885 > YA/2 + 7
Solving for YA, the inequality becomes
YA > 23.77
So, if you are 23.0 right now, and she turned 18 six weeks ago, you must wait until you are 23.77 years old in order to date her. That would be at about the beginning of November.
Or, you know, just ask her out now. From your comment about working up courage in the OP, I get the feeling that you’re using her age as an excuse to avoid asking her out. My 29 year old brother dated a girl that was 24, the same age as my sister, and our family didn’t so much as raise an eyebrow. Give it a shot.
Hmm. According to that rule, I can date a 29-year-old.
Great! 
Do it. My roommate’s 26 this month and he’s nailing an 18 year old.
Also, does she by chance wear pyjama pants to class?
When I was 29 I went out with an 18-year-old. 
The relationship was short-lived for a number of reasons, but the age difference wasn’t one of them.
Dude, stop talking to teh intarwebz and go get her already! There will be chicks your sister’s age. You will either get over it or you will regret it later.
Awesome.