Help! i'm confused! literally!?

Recently i found the love of my life, he's a wonderful guy, and we might get engaged soon... the only problem is, that i didn't get to meet his sister before except 3 days ago, and what a surprise...i found out that she used to be with me when we were student at the university, n i used to be such an A$$ with her, really mean...so since i didn't meet her face to face yet....i don't know how to act toward her, but i know that i regret being rude and childesh...another confusing thing...the resemblance between my boy friend and his sister is easily noticed and now when i look at his face i see her! and it confuses me! perhaps what i'm saying doesn't make sense...but the weird feeling is there! what do u suggest?