I have to agree with others and the post below having a sister and brother examined together. When they are younger, I have no problem being examined together. Once a sibling reaches 11 or 12, they just need some privacy.
Everytime i went to the doctor it was usually horrible and a anxious time. My older sister and I used to be examined in the same room until I was 14 or 15. When I was young, I did not care my sister and sometimes her friend were in the exam room. But once I was 11 or 12, it was embrassing just a horrible experience having my sister watch my physical. The following is some of my worst experiences being examine in front of my sister.
All the examines started the same way, with me stripping to my underware in front of my sister, mom and nurse then sitting on the exam table. The first thing the doctor would do was to pull the front of my pants down with the comment " let's see how the little guy is doing today". Later, on having your foreskin checked/retracted in front of your sister. I don't know any guy that wants his penis referred to as "little guy" or having your foreskin/ penis checked in front of your sister. The worst/last physical with this doctor came when was 14. The exam room was very cold and I suffered some "shrinkage" while waiting for the doctor. Long story short, he wanted to recheck my penis/testicle so before I knew it he pulled my underware down below my knees. There I stood there in front of everybody nude while the doctor examined my testicles and penis.
I just think some privacy is need for both siblings when they are being examined as they get older. I was never present during my sister's physical, after my exam was done I had to wait in another room. It must be a little awkward for the one sibling being present during the others physical even if both are sisters or brothers.