I have just finished reading 'The Making of Us' by Lisa Jewell, which I really enjoyed.
The basic plot involves three young people -- Lydia, Dean and Robyn. All three were conceived from the sperm of the same sperm donor and, as the story unfolds, each learns that they are a sperm donor child. Furthermore, they each learn, via registering with the Donor Sibling Registry, that they have siblings. They are very curious about this and, in the end, they arrange to meet one another.
Despite the fact that they are all very different personality-wise, when the three meet, they immediately connect on a deep, emotional level. It is as if something was always missing from their lives and now they have found it. They are at ease and relaxed in one another's company, right from the word go. Their common heritage appears to override everything else.
It was this part of the book that I found really interesting -- the immediate connection despite the enormous personality differences and the absence of contact previously. I don't know whether that is true to life for donor siblings, but it is rather different from my experience of (non-donor) family. Having grown up in a very difficult family, shared inheritance -- the genetic connection -- doesn't mean a great deal to me. What matters from my perspective is that someone is a good, decent person who shares my values and understands my way of thinking. That may be a family member but, equally, it may not. There's no hard-and-fast rule: it all depends on your own personal experience, I think.