I totally disagree with your comments about good peeople telling them. My wife adopted 2 kids in her previous marriage - both kids do know they are adopted, and the eledest son (25) would be devestated if someone got into contact with him and wanted to meet him. As far as he is concerned his mum (my wife) is his mum and thats it. That said, she is aware of other couples who have decided (rightly or wrongly) not to tell there child.
I am assuming you are 29 - You say it happened 19 years ago whn you were 10. If this is the case, how would you feel if your mother suddenly turned around and said "by the way you are adopted". WE didn't tell you as there was never a good time (firstly you were too young, then there was all those problems with your sister, then it seemed too late, but now a member of your original family are trying to get in contact with you.....
Doesn't sound too good does it. Yes he may know, and in which case he may want to trace you. It will be very easy for him as his parents would have his original birth certificate with your sisters name on it. His parents would also know the area she came from. If he wanted to, he really could find you.
Everyone makes there choices and have to stand by them. What happens if he is in a good situation. What if he is ok but then gets devestated by this news.
I really do think that the past is the past. If this is a really pressing problem to you and or your sister I really would suggest councelling