I know a few people around me who was adopted at birth or at a very young age. I have also come across a few clients who informed me that they were adopted. Obviously it is not an uncommon practice and I personally think it is a wonderful system that unites parents who are willing to raise a child and children who are in need of different parents.
I have never been on either side of the adoption so I really cannot speak for them. But I often get a feeling that they sometimes do wonder about their birth parents, especially when they have had some difficulties with their current parents. They may be wondering if things would have been better or different if they were with their birth mother, for example.
While it is absolutely natural for anybody to feel that way, spiritually speaking, I feel that at least for your soul's evolution, it really doesn't matter who your birth mother is, if you never see her in this lifetime. On the other hand, your adopted parents may actually be more important in that you get to spend a lot more time with them and therefore, there will be a lot more physical and emotional exchanges, which is most likely to be part of your life lesson in this incarnation.
Even though I wasn't adopted, I had some difficulties over the years in the relationship with my own mother. I am sure I am not the only one. When you are young, your mother or parents are much "bigger" than you. They are there to "teach" you. But as I grew older and learned more about life and spirituality, I came to a realization that just because they came to this world and assumed their roles as your parents, they are not necessarily spiritually mature than you. Sometimes you are here to teach them some important lessons by coming into their life as their child. With my own children, I also try to maintain this awareness that I am temporarily taking a role as a mother. So while I teach them things like mothers normally would, I never consider myself to be superior to them.
Anyway, going back to the adopted family relationship. I believe these people are meant to spend life together; meaning they'd agreed to meet in this life before incarnation and there are life lessons to be learned from that relationship. So, whether they are blood relatives or not doesn't have so much importance, spiritually speaking.