Horatio I think you are asking about one specific thing, or are you asking about two quite different things?
Here is my best 'suggestion'....
If your main concern is your friend and her child you could talk to your friend and see what she can suggest and you could research this specifically with the country in mind. I can't begin to know what the legal restrictions are in that country but if it were my friend and my country (and my friend did die and left a child the same age as mine so it did briefly go through my mind)...I'd get all the facts and details straight.(Remembering that there may be exceptions to the general rule due to some things).
In the UK you can make some kinds of arrangements, I think, so I would explore this with her and see what you can put into writing if the worst comes to the worst. She may have other plans, she may be pleased to discuss it. I just feel it is best to be out in the open. We have it in our wills (DH and I) that if we both die my sister will take our kids. This is to avoid any uncertainty from anyone in the family.
The order of child ages does not matter so much when you are taking in a child known to the family, very much like step kids - who may be all different ages etc and fit in OK. And may know each other before they become siblings or step-siblings.
In terms of adoption from social care, the order is normally birth order and this is for good reasons. This is for protection of all the kids.
The birth kid who may be less troubled and less 'street smart' than the new incoming child, and a physically larger and perhaps angry/stressed new sibling will not be nice for a birth child.
Also for the incoming child, who will almost certainly have more needs, these can be better met and the child will be better cared for if that child is the youngest in the family (or in some cases he only one in the family a the time).
I think if you are looking to adopt as an altruistic approach then unless you wish to take a very troubled or disabled child you really do need to re-think. The easier to adopt, fewer-issue children will be in 'high demand' if I can say that without it sounding weird (and yes I am native speaker!)
Many who adopt with birth children (like me) were not able to have as many birth children as they would have liked. Although, personally, we really did also want to adopt so there are often a whole range of mixed motives and mixed emotions.
But purely altruistic is, I think, potentially problematic for adoption. And not generally a good reason, alone to become a parent. It's a bit like the difference between befriending someone because you want to be their friend, and because you see them as a 'project'. Of course, if a person has no friends at all they may be pleased to be a 'project'.
But ideally everyone deserves to be be-friended and loved for their own self and this is true of children too.
If that makes sense.
Good luck. You sound very kind and I think you need to work out exactly what it is you need to find out. 