@1966adoptee Yeh I know what you mean, I was raised with a brother who was also adopted from another family. We have nothing in common and although wet on ok and so on, there is no hint of a bond to speak of.
Fast forward to 2016 and I get contacted by my big sister after 42 years (last seen me when I was 10 months old) She is in the same position as me. Raised with an Adopted brother but no closeness etc. We started by just chatting. About everything and all things. I once asked her how to describe the bond we had started feeling, she told me "It's like talking to the voice in my head."
We also found my bio fathers boy and girl. For various reasons my half brother is the youngest but really has had to take the roll of oldest. Now he has me it's like he can be the youngest again and frankly when we get together its really just us both making fun of each other and unrelenting teasing :P
I'd say don't rush it. Let it happen and sometimes you will need and need to give space to the other side too, after all they are learning this new dynamic too! AND some poor soul is now having to adjust to YOU being the oldest and that they (like myself) are now a middle child! hehe!