I was very surprised to see halfway through the post that these are adult children. I'm not sure how you are getting their mail. But they need to see these cards and need to be told that their uncle (1) may try to contact them, since he already has done so; (2) apparently has emotional and/or mental issues, or family history, that make him hostile; (3) it's their choice as adults whether they wish to contact him, but you believe having contact with him would only make him feel free to be more hostile and hurtful (if you do indeed believe that's true).
Then, please!, you need to apologize to them both -- a lot -- for intercepting and opening their mail, and you need to promise never to do it again. Do you see how doing that, even in the name of protecting them, can destroy their trust in you? You did it from a basically loving impulse to protect them, but because they are adults, it would be understandable if they now were very wary of you and wondered: Has she kept other mail of ours from us, not just these cards? Has she kept other family information from us? Even if you have not -- can you understand why they might feel this way, even though you were trying to protect them? You did mean well; tell them that; but also apologize if you want to keep their trust. This includes the sibling who did not get a card, by the way.
You need an honest and open discusssion about why you are so afraid of your brother that you are protecting adults from him in this way. Your kids may be able to help you; they should be there for you to lean on too, not just there for you to protect; and you and they should be able to talk together like adults.
One other thing to consider. Is it possible that your brother is mentally ill on some level? You say you haven't seen him years so you may not know what is going on with him. There may be a lifetime history of his being a bully and if that's the case, then yes, he's just a bully; but IF he was a different person at one time, then it's possible he's mentally ill. My husband has an aunt to whom he has always felt very close, and she is usually a very lovely, kind-hearted, gentle person -- most of the time. But at times in the past 15 years, she "goes off" and writes notes that accuse people of talking about her, plotting against her, doing cruel things to her that never happened at all. She also sometimes goes off at family gatherings, accusing her sisters of doing things and plotting things and stealing things--all in her mind. We used to have fun with her and now the visits are tense because we can't know from visit to visit if that day will be a happy one or a day where she decides one of us is telling all her friends lies about her. It is unspeakably painful, and we can't convince her yet to get the help she needs. But just bear in mind -- if your brother was once consistently OK -- that he might be undergoing a mental illness. That does NOT excuse these cards but it may be an explanation that could frankly help you ignore them better.