Our daughters, while still young, know who their mom and dad are (us), even though they know they were adopted and they know as much as we can share with them at their age. They are also the best of sisters.
I once worried as you do. I can't begin to describe the special comfort I received at the sealing of our first adopted child. Your children will be your children, no matter how they enter your life.
Our "open" adoption consists of sending pictures once a month during the first year, and then once a year after that. We also send small specialized gifts from the child every now and then. I felt impressioned to let them meet with her a year ago, and we did so over lunch. It went well for all involved.
Our second adoption was also supposed to be similarly open, but that birth mother quit responding within the first year. We have had no contact with her since. Her choice. This actually saddens me, as I know my daughter already wants to know at least a little about her birth mother, but we have very little to tell her.