Birth daughter wants to meet me (birth mother) and her half-siblings. How/when do I tell my kids about the adoption?

I’d like to start off with some background info. I first gave birth over ten years ago. I was 17, in high school, and in no way ready for a baby.When I met my birth daughter’s family I knew immediately that it was meant to be. We decided on a semi-open adoption. They would send pictures and letters the first two years. After that any contact was at their discretion.We continued contact after those two years. It’s been better than I could have ever imagined. Her mother and I are connected via social media, we text, and when we talk on the phone it usually goes for two hours or longer. I have full confidence in the decision I made as a teen. They’re a wonderful, loving family.Since the adoption my life has changed a lot. Relationships came and went, I moved across the country, got married, had two kids of my own, divorced, and remarried.My birth daughter has always known about her adoption. Her mother has shown her pictures of me and she knows she has two younger half-sisters by blood.Birth daughter wants to “meet” me and meet my daughters. (I last saw her when she was about 3, but she doesn’t remember that.) Her mother is fine with it, and I would love to see her again.My kids don’t know they have a half-sister who was adopted. They don’t share a father, they’ve never even heard of birth daughter’s biological father. They just... have no clue.I haven’t told them because it’s a big concept for a kid to wrap their head around. I’ve been afraid that they’ll think I “got rid” of my first daughter because I didn’t like her or didn’t want her or she wasn’t good enough, etc. I’ve been afraid that they would start to fear “what if mom doesn’t want me, either?” Kids just aren’t always rational, and I didn’t want to plant seeds of doubt or fear in their minds.I wouldn’t take them on this visit without them knowing either. Birth daughter is still just a child herself, and it would be very likely that she would say something about the adoption to my girls. That’s not how I want them to find out. But I have no idea how to tell them this, or if the near future is even the right time.But as the years tick by, the day will inevitably come—one way or another—that they’ll learn about my birth daughter. I don’t want that to be a moment of betrayal for them. I want that to be a happy moment of “wow, our family is even bigger!” (Their Dad is remarried. They have lots of biological and step-family members now.)Does anyone here have experience of learning that your parent had a birth child before you? Any other birth parents able to give advice on what the right time, age, etc might be? Or anyone with just some sage advice? I don’t know how or when to give them this news that won’t radically change their views of themselves or their “role” in the family. via /r/Parenting https://ift.tt/2qP7Wb9