HI I was 16 when a sister I didnt know existed appeared. My mom had placed her for adoption 11 years before my birth. I think that you should be aware that if you dont tell them now there is a chance your daughter will just show up and that would be harder! My mom sat us down and simply told us the truth. Explained the shame and situation as to why she hadnt told us. What shook me up is that maybe I didnt know my mom as well as I thought I did ... but overall we asked lots of questions which she answered. We were concerned about what "our" dad thought about this - and he was great which also helped us! It was hardest on me - I lost out of being the "oldest" the first grandchild etc... all those things which had been important to me growing up HOWEVER saying that, I dealt with it fine, loved my new sister. Just do lots of reassuring ... explain that your love for them doesnt change, their place in your heart doesnt change. Sadly, I also needed reassurance from my mom that she wouldnt "leave" me or cut me out like she had my sister (this all from the perspective of a teenage of course) which she gave me. Kids dont look on these situation with the same emotional baggage that us adults do - they are liable to be very curious and want to search for her right away
good luck and let me know how it goes! Jen