I agree that emotions in adoption do run very deep. I don't think it's right for any adoptee to face judgement for making the intensely personal decision to trace, or not to trace. But I also know that you can't help your immediate emotional responses to this, and I also don't think it's right that you be judged for having a strong emotional response.
I think it would be very unreasonable to say anything to your brother about this, because he isn't doing anything wrong and he can't change the way he feels. But I'm not going to say you're unreasonable for feeling this way in the first place, because we can't help our deep down emotional reactions to hearing news like this.
I can see the parallels with my own family actually - I have two adult children, one who has close contact with her birth mum, and the other who has no birth parent contact at all. Although they both see their (birth) siblings. But when it comes to being in touch with birth parents, they think very differently and I know they don't emotionally understand the others response, which has made for difficult conversations in the past.
Whilst you can't help feeling 'inadequate', most adoptees who trace are not doing it because they want another family, and they aren't doing it because their beloved parents and siblings aren't enough. Your brother isn't wanting to replace you. But sometimes there are things people still need, whether that be information, or answers to questions. I guess the way I see it in my head, as an adoptive mum, is that I play a different role to the birth parents and they play a different role to me, so we aren't in competition. Not that that stops me from getting insecure sometimes, or having little emotional wobbles!
The important thing is whether and how I act on my feelings, not that I feel them in the first place (IMHO)
So...do you feel you can offer support to your DB if he wants any support from you? Even if you are feeling conflicted inside, can you say the right things to him, and not make him feel guilty or upset?
Maybe it would help to talk it through with someone?