I have read also some relations of physicals in which a boy is examined with his sister present in the room and generally the mother but the reverse is not so common.
So it not only to reduce the cost of the examination of two children in front of a doctor or because they are twins which would be more logical.
What is surprising is that none the doctor nor the mother seems to consider that an older sister or a younger sister cannot stay in the exam room during the examination of her brother younger or older if it is performed in the nude or partly in the nude because it is embarrassing for him, while they said to the boy to go out of the exam room and to wait in the hallway when it is the time of the sister examination.
Nobody seems surprised by this behaviour and this disrespect of the modesty of boys.
of course, it is also depending on the age of the children, under 10 age, I don't think that it is a big issue, after it depends on teenagers and if they were prudisch at home.
I am interesting by the opinion of doctors or mothers.