Hi - I was 16 when my mom told me she had found a "new" sister ... one that she had placed for adoption 11 years (27 years before) before my birth. WOW. What a shock ... I did go through some emotions ... but mostly was concerned for her ... she really let us know how painful it was for her and why it was so important for her to know her "first" daughter. I wanted to know why she had kept it a secret and WISELY on her part she said that it had nothing to do with me or my character but simply because she had been so damaged by the process all those years before she hadnt had the courage ... made me feel a little better about my mom not trusting me with the info. She was as honest as she could be with her emotions and included us alot. Some of the emotions we experienced, which your children might as well, was a bit of jealousy - I enjoyed being the "oldest" and my mom was so keen on including her first daughter in our family that I promptly got bumped from that role - lost out on first grandchild etc... (Got over it quickly though!) Be prepared from some jealousy and insecurity especially as usually at the beginning of reunions everyone is sooo "in love". Maybe ensure (which I know is hard as you will be dealing with your own emotions) that you spend some time without the new daughter with your existing children ... allow them to vent etc... Hopefully it should be easier for you as they are more mature. Maybe also want to address their fears about "inheritance" (some fear that the "new" is out for money etc... and that you are committed to this child ... no matter what.
God Bless you on this journey!