I'm not sure if this is the right forum to ask for advice in... but I've been lurking off and on for a few days, and if there's one thing I've learned, it's that you ladies do brutal honesty very well. I think I could use some of that right now. 
Here's my situation. I apologize in advance if this gets long:
I was adopted.
My parents are very sensitive about the fact that I am not, biologically, “theirs.” They have always kept the fact that I was adopted a secret; I didn't find out until I was 17, and only then because I'd put two and two together and demanded the truth.
A few weeks before my 18th birthday, my Dad took me aside and said that if I was curious about my birth family, he hoped that I would keep it to myself. “Do what you have to do,” he said, with honest-to-God tears in his eyes. “But, please: Don’t tell us about it. We don’t want to know.”
Bewildered and a little hurt, I promised him I wouldn’t -- and didn’t.
So now, 10 years later, I have not only met my birth mother and her entire extended family (which is HUGE!), but also consider her one of my best friends. I didn’t go looking for her: knowing how my parents felt about the entire “adoption thing,” I wouldn’t have done that even if I had wanted to. But she tracked me down a few days after I turned 18, and -- after avoiding contact with her for over a year -- I realized that I really wanted to talk to her, if only to thank her for giving me my life.
It turned out to be one of the best decisions I’ve ever made. She's become something of a big sister to me, and I feel very fortunate to have her in my life.
The problem is that my parents still don’t know, and don’t want to know. I’ve tried so many times to talk to them about the adoption, but they inevitably shy away from the subject -- sometimes angrily, and sometimes with such obvious hurt that I just have to let it go.
I’m currently planning a wedding (obviously, since I’m posting here!), and I would love nothing more than to have both of my families in attendance. I've already asked my half-sister to be a bridesmaid. I figure I'll just introduce her to my parents as a friend. She has a different last name from my birth mom, and we don't look at all alike. I doubt my parents will see the similarities between us, especially since they won't want to.
Still, I feel awful about excluding my birth mother -- who has become a significant part of my life -- from my wedding, and the idea of continuing to lead a double life is absolutely exhausting to me. What happens if FI and I have children some day? How do I continue to separate my families then?
For the record, my birth mom is fine with missing the wedding. She says she loves my parents for giving me the life she couldn't, and doesn't want to hurt them any more than I do. Her kids are a little less understanding, and my 22 year-old half brother is especially hurt ("Why does {sister} get to be a bridesmaid when I can't even come?"). (It doesn't help that I'm a bridesmaid in his wedding, which is just one month before mine.)
Any thoughts you have would be appreciated. I don't think I'm even capable of seeing this objectively anymore.