Thank you all for the replies. I forgot to mention in my OP that my DH is on board. He may not have thought about it as much as I have, so counselling makes sense, and it will probably have to be online after the children are in bed. I wouldn't do this if he were unhappy or had concerns.
Maybe it's easy to say now, but I don't feel in any way that this would be my baby. I trust my brother and sister-in-law will be great parents because they're already a great aunt and uncle. They won't make all the same choices we do, but that's fine, because it's their child. I can’t imagine feeling an ownership of their parenting decisions years down the line because they needed help being their baby here. I also don't think I'd hugely struggle to hand the baby over. With both of my children, it took weeks to months to fully bond. I loved them from the start, but I needed time to get to know them. It wasn't PND, and I had immediate skin-to-skin and breastfeeding both times. And that was without knowing the baby was my niece or nephew and would never be coming home with us. Of course it will be emotional but not like handing over either of my children now. The same goes for the comments saying the baby would really be mine. I'd carry and give birth, but the baby wouldn't be biologically mine and was never going to be. I wouldn't have given away my children because I knew they were mine. This child would never be mine.
I care hugely about the baby as part of this process. The data on outcomes for children born through surrogacy is not well explored with no large studies. I think my niece or nephew would have an amazing quality of life, with parents who adore them and doting grandparents, aunts, uncles and cousins. Even though I'd be the one pregnant, the baby isn't mine to "gift", and I actually feel uncomfortable with that word being used about another human being. The baby would be carried by me, then grow up with their biological parents. I was very interested to read Beccahm's posts, and I've a lot of posts with a wide range of experiences from surrogate born people, positive, negative and neutral. There doesn't seem to be any consensus, or any large scale, long term studies.
I do acknowledge there are risks, but there are risks with any third pregnancy too. People also have more than one baby from donor eggs not infrequently. I'll definitely do counselling, and I'm sure this will come up there.
As for my children, we’d be honest from the start. I'd tell them that sometimes people struggle to have a baby and need help, and that I'm helping Uncle X and Aunty Y have one. I definitely wouldn't say I'm having a baby and giving their brother or sister to Uncle X. In my experience children are very accepting. We have books at home about different kinds of families, and they already know many families that aren't Mum, Dad and two children.
With regards to disability and termination, if I decide to offer and they accept, that's a bridge I'd cross then. Personally, I would have terminated for a serious disability, but I don't know what they would do. I can't imagine them rejecting their baby if they were born with a disability.
The advice to speak to my brother first makes sense, and I'll take it on board if I decide I'm willing to go ahead.
What if I miscarry is something I’ve considered but it’s something I can’t really say how everyone involved would feel. Similar for siblings, though my thought now is I would only be happy to carry one more baby.
We would be on the initial birth certificate but a court order would reissue the birth certificate to reflect the biological parents.
I hope that answers most things. I’ve found every post and POV helpful even the ones I don’t agree with. This is a huge decision and an anonymous sounding board is incredibly helpful.