Hi @Highfivemum a few years ago I was contacted by a man claiming to be my brother. I was gobsmacked because he was two years older than me and it didn't make any sense. My birth mother, who had me adopted, told me a story of being alone, pregnant, scared and desperate but she had been through it all before, literally not much more than a year between the time he was given away for adoption and she got pregnant with me. I was very reticent but so was he, and we sent a few emails and left it at that, maybe 3 a year. Then last year I was on a weekend away and realised it was near his town so I asked to meet for coffee. It went really well, he has exactly the same mindset as me and has a busy full life. He was brought up very similarly to me, in a loving family with lots of aunts uncles cousins etc and we have enough people to be keeping in touch with without drawing more drama on ourselves. We also have a similar relationship with our birth mother. That is, I text her occasionally, we sent Christmas cards and I've met her 3 times I think. I sometimes randomly send her messages about medical stuff but very casual like - hey did you ever get cysts and she might reply, no but my cholesterol is going up, you should know!
My mother went on to have 4 more kids after me and my birth father had 5 so there are 9 more out there that I've never met or showed interest in meeting. I've never even looked them up online although I know their names. I could drive a few hours to their town and I'd probably bump into a few, they all live near each other but are unaware of their connection. What's interesting is my 'new' brother and adopted sister have the same blase attitude about biological family. Its funny but I think when you grow up without blood relatives you don't value it as much, you'd think it would be the opposite but I just don't associate family with DNA.
The big difference obviously is you and your DB went through a lot, it sounds like there was childhood trauma and massive responsibility on your part. So its bound to be much more complex. My situation is simpler and less emotional. I feel occasionally guilty that my half siblings, particularly mothers 4 kids were not told about me, I think its shitty she didn't tell them but I'm happy to not get involved. Sorry for the long post - but as others say be truthful but tread cautiously, very cautiously until you know what this sibling is about and don't be afraid to be selfish and say no if needs be. All that said, this person could be a blessing to you too, you never know. Very best of luck to you and your brother.