In her defense, the school panhellenc might have a bit of an inferiority complex and think that enacting some rush straight out of the 50's will make them cool, then the sororities may be fully normal, non-pearl clutching girls who have to play the game for a week a year. And then go back to dressing like they live in some era approximating NOW.
If, however you do in fact go to an incredibly stuffy pretentious school (which is the only way I could describe that ridiculous dress code), then sorority life is probably not in the cards for you.
Regarding the issue with trying to find a bond like your biological sister, it is not going to happen. Ever. And you shouldn't be trying to find that. What you CAN find is a core circle of friends who are there for you in a pinch, understand your every day battles and can make you laugh through the tears. And lend you shoes
What I would suggest is do both; look at informal rush AND look at the colony. At some point you will have to make a decision and you may not have all the facts before having to do so. As soon as you find a situation you're happy with, if they offer you a bid, I would suggest you take it. There is not a best or worst, right or wrong; there is only where you feel comfortable and welcomed, and very likely that will be at more than one place. So don't be that person who loses all opportunities because she can't commit to any. Best of luck to you!