Hi, I was in a similar situation.
I was in contact with my bmom for a while which was a complete disaster, long story, but before I contacted my 1/2 siblings I did write to her to let her know this is what I would like to do, I thought letting her know was the right thing to do, she never answered my letter so I did contact them.
I wrote a letter to each of them and they were so happy to hear from me, 2 are a little torn but have said they are happy I contacted them.
The thing is, I didn't want to be an old lady looking back wishing I'd reached out to them, that would be so sad. Even now though, being in contact with my siblings, there is still quite a bit of pain involved, so many emotions etc but I'm glad I did it.
I thought I would never hear back from any of them and really thought about the pain I would feel if that happened but I was in total pain anyway knowing they were out there and had no idea I even existed.
I would definately contact your siblings, after letting your bmom know this is what you need to do, at least you are giving her the chance to tell them too, I'd advise not going through anyone else, write directly to each of them, again I know from experience going through another person to contact them can be a disaster, people sometimes feel they are in charge and get in the way of what should be a reunion between just you and your siblings, it can cause all kinds of problems that should not be happening.
They need to be able to decide what they want and you just never know how someone is going to react.
I absolutely agonized over writing to them, what if they hate me, what if they don't want to know....but in the end it was really my husband who said, If you don't send the letters you will never know what would have happened and at the very least your brothers and sisters will know you exist, so off to the post office I went!!
It was a huge relief to mail them!
I wish you so much luck in whatever you decide, I know it's scary, by the way one of my 1/2 siblings told me she would have searched for me if she'd known I existed! You just never know what is going to happen, take care and look after yourself.