H.D.
This is one of the hardest talks to have with kids. So many times there are hard feelings attached to break ups and it is hard to be objective and not put your own feelings into it. But the best answer is stay to the facts.....
Dad and I weren't a good fit. Sometimes that happens. It is sad but it is what it is. Our feelings towards each other have nothing to do with you, we both love you and you are the best thing to have come out of our union. All kids of divorce wish their moms and dads would get back together but it is not realistic and won't ever be happening with dad and I. Dad is getting remarried and I really hope that this time he finds the partner in life. Do your best to be kind to your new stepmother. Hopefully in time you will develop a good relationship with her too. Dad and I love you and that is what is really important.
As for your "right" to know the new wife? Unfortunately you don't have any choice as to what kind of woman he picks and whether she is in your child's life or not...unless she has a criminal record but that will mean a long, drawn out battle in court. I suggest you take a realllllly DEEP breath, give her a call, ask her to meet you for coffee and get to know her. If that seems too much then try to talk to her on the phone or via email. Believe me, she is just as nervous about taking on your daughter as a new step child as you are letting your daughter go. Get to know her, be willing to keep the doors of communication open. Your daughter will benefit and you might too.
As a stepmom with an angry ex-wife of 8 eight years I can tell you that you have the ability to make life hell for your ex-husband and hurt your child with that anger. I would not chose that if I were you.
I am also the ex-wife and my kids have a step mom. Instead of being angry and evil I swallowed my pride and put out my hand. Oddly enough she and I are now great friends! Since I was with her husband for 20 years I know some things that she doesn't (they have been together 5). I have encouraged her, counseled her about my children and am very happy that she is a co-parent. My children know how we feel about each other and know that they have 3 parents that love and support them.
A lot of it is choice....yours. Your daughter lost her bio family but she has the opportunity to have more. Help her through it.