I think it could be a good idea to talk it over with someone- adoption counsellor, for example.
You do have to accept that if they don’t know about you that you are potentially going to turn their world a bit topsy- turvey and they may not thank you for that. You may be fortunate and it all works out well but I think you need to be sure you can deal with whatever response you get.
I know someone this happened to and it led to a lot of heartache all round and I know she has been really hurt by the fall out (and I know it has been hugely difficult for her other siblings and badly affected her mum). Given what has happened I suspect her half-sibling hasn’t found it easy either. I know my friend really wishes her half-sibling had not made contact and there is a huge amount of resentment, from what I understand. It’s quite a sad situation really, but I know my friend feels like she has had a grenade thrown into her family for someone else’s perceived benefit, with no say at all.
If you really feel the need to contact your half-siblings and are fully prepared to deal with a bad outcome as well as a good one, as well as being very much aware that you could be causing upset to others so tread very carefully, then I suppose you could consider contacting your half-siblings.
Maybe you should let your birth mother know you intend on doing this first? Perhaps she would prefer to tell them, even if forced to do so by your decision to contact them, rather than them finding out from you. I’m not sure if that would be better or worse, but something to consider?