Should I tell my 18-year-old sibling that he was adopted?

I honestly dont think its your place to tell him.
its sad his parents didnt tell him as he was growing up. I think parents should tell their children they were adopted they have rights to know about their background. IF i was you i would maybe contact his parents first and let them know you would like to meet him but you are worried because he doesnt know he was adopted and thought maybe they should tell him first.
Itts truly not your place to tell him.

well, at 18 he is old enough to make the decision of whether he wants contact or not...I personally think that they should have told him. But, that is not my decision and its not yours neither...try contacting his aparents. Communicate with them first...see if they will tell him and all that and let them know you want to make contact.

I have a different take on it. I believe he does have the right to know. What happens if one day in the future something very serious medically happens to him or his children (when he has kids)? Who would he turn to for assistance if he needed information or something more invasive like bone marrow? Usually you start with your family. What if this guy gets screened for testicular cancer but not prostrate cancer? Basics. Also knowing his true history and his complete identity is important on a human decency level. I would tell him. But also know that usually it is the messenger that gets shot. He may want a relationship with you. He may be angry. Duh. Goes without saying. But you can't control who he will be angry at. You can only give him the gift of truth. In the most compassionate way. Give his parents a chance by telling them you intend to spend more time getting to know him. Let them do the math.

I agree too, he has a right to know and if he is eighteen and his aparents haven't told him the truth then the are risking the fact that another will be the one to do it.
With that said though, I think Allaboutkeeley's suggestion is a good one. Send his aparents a letter and let them know you want to meet your brother. The very fact of learning that his bfamily has found him might very well spur them into telling him the truth and make them realize this is a secret they can't keep from him forever.
They can't deny you from meeting your brother, that is your right as siblings but atleast they are getting a chance to be truthful with him before you two have contact andk maybe just getting that kick in the butt Allaboutkeeley mentioned to finally tell him the truth themselves as it will be better to come from them because he will find out at some point, I don't doubt that for a minute.

I tried that but they said once the adopted him they became his family. See they are a little bitter about the whole thing. I dont know what else to do. =-(

Don't tell him, be a good sister and let him be happy. Just be happy knowing he's happy

If the parents will not tell them themselves, then I think you should.

He has a right to know his medical history and to know that he has siblings out there that care about him. In no way does this discredit the family that raised him.

My friend was adopted and all of the siblings met and became friends as adults. He still calls his adoptive parents mom and dad and his siblings with them brothers/sisters..... but now he has these other connections that are special too.

Good luck....I hope you get to meet him. Just don't have too high of expectations bc he will likely be upset to find out his whole life is a lie.

I also agree that he has a right to know his truth. Don't be surprised if you end up being called the bad guy. Just know that you aren't. Shame on his parents for lying to him. The secrets and lies in adoption are unnecessary and harmful.

yes then I would be happy! LOL my marents and I don't get along at all

There is no way to predict how he will react to the news and you need to prepared that people will blame YOU if he takes the news badly. However, if his adoptive parents had told him as they should have, you wouldn't be stuck in this difficult situation. Therefore, whatever happens when he is told is THEIR fault, but, again, just expect that some people may unfairly blame you.

You do have a right to know each other, and if your brother's adoptive parents don't tell him about his adoption someone needs to do so.