Salam
There seems to be a confusion of semantics and so many different situations.
Many people who adopt do so because they cannot have children of their own - therefore, a couple could "adopt" (or sponsor, take-in, foster, whatever word you want to use) a newborn baby, but if breastfeeding is the only mark of making the baby part of the family, what do you do when you can't breastfeed? Heck, the majority of Kuwaiti women don't breastfeed their OWN child (it is much more popular with the Western women in Kuwait than the Kuwaitis) let lone thinking of taking in another child to breastfeed.
Seriously, if I were already breastfeeding a child (as in, I have a baby), that would be the least likely time when I would think of taking in another child - not because I am a mean person, it is just that we all know that a child is a LOT of work and can be very tiring and physically exhausting.
Yeah, and what if you adopt (foster, sponsor, whatever) a child that is past breastfeeding age anyway but still young (say, like a 3 year old)? I honestly can't believe that anyone who takes-in a child and raises as their own child would ever think of marriage with said child (and if the child was rasised as a sibling - kids don't think about marrying their siblings).
If you take in and orphan, do they share in inheritance? Otherwise, what will they ever have for themselves if they don't? I have heard that they inherit from their parents, but many orphans that are available for placement with other families come from poor families (as in, if their parents are dead, there are no extended family members that are able to care for them either). So, what becomes of them in the future?
I don't know - I my opinion, it seems more confusing the Islamic way than the Western way - when a child becomes legally just like a birth child, with all rights and responsibilities that go with it. Perhaps the western way can just be melded a bit (as in, the birth certificate has to keep the original parents names, but have a special certificate to indicate who has the legal rights to the child)?
Lana