couple things first... thread moved to the secuality section because this is, indeed, relationship stuff. second, asu.. you live on campus or off? i went for a year and lived in best hall. good times.
onto the advice. it sounds liek you both have some hurdles to get over if it's gonna work. if you're willing to put the past behind and she do the same, it'll be fine. i've had too many problems with girls who keep "intimate" ties to their past... makes for bad news. as to your condition, don't sweat it. if she really loves you it won't be anything to worry about.
now about her sister. you're treading on very thin ice. you need to make sure that nothing like that ever happens again if you really want a future with this girl. mind yourself and your actions.
overall, i say enjoy it and don't put too much thought into it... if it's meant to be, it'll work out.