I have a sister which was adopted to my family. Her birth number is 9 from his bio family and 6 from his adopted family. It seems that her birth order always follow the adopted family. Even though I had tried to update her birth order from her bio siblings, it will be auto corrected to follow adopted siblings. I had also tried to clear her birth order from her adopted family but still fail to update her bio birth order.
I do believe this is something was not carefully designed and implemented during adoption feature release.