the siblings question is an important consideration. I don't think doctors and other med pro's will approach this subject for non-siblings because of liability. but as soon as I saw the question about 2 non related children, I immediately though about people in close proximity for most of their time. and the siblings issues can shed light on this. here is my thought thread on this. If I share a razor blade with a stranger, I can get infections on my skin that are harmless to the other person, but everyone has different tolerance to bacteria and other pathogens. however, i can safely share a razor with my wife. why? A doctor shared with me that we all have a variety of pathogens living on the surface of our skin that are completely harmless to us, because our immune system handles it. After living with my wife for several years, the sharing of things, that was once risky, is now safe. Over the years, our immune system becomes exposed to each other's set of pathogens, and can prevent any infection.
That is part of what happens with siblings. although a similar genetic makeup also can add to the safety factor. However, if your family members and your children have been close since the baby was born, they are already sharing pathogens. And at such an early age, the immune system is still developing. You may get the same benefit of a husband and wife and other family members sharing things, but you should not take my word as any level of truth. The point is: talk to medical people until you can find some who will have totally honest conversation with you about this issue. then report back on this forum. I am sure that many others would like to know more.