I have no sister but I am always surprised to read some of the relations about that subject.
Of course I know that especially in a recent past, 60's or 70's years, there was a double standard towards shyness towards nudity for boys and girls
Most of the fathers did not think that a feeling of shyness towards nudity modesty towards nudity was a good feeling for boys, while on the reverse they might learn to be naked in a group of boys as it would be later at military service, or in front of adults of both genders, it did not mind.. and many mothers thought in fact the same, even if they were more concerned by the feelings of their sons.
Of course most of the doctors, male or female, and female nurses did agree with that opinion as well as school authorities and it had consequences on the set up of the school or college or sport examinations. A boy was never offered a gown at that time and he receives sometimes an instruction to strip completely for all the examination normally to win time but more likely to toughen him or because it seemed more practical or just by customs in that medical office.
On the reverse the modesty of girls was more considered and respected and I don't critizice too much that difference because I think that there was some good reasons if not all for that situation at least at that time.
Even if we boy's students had no first hands informations at school, since it was not mixed, we knew by rumors and by those who have sisters that the physicals examinations of girls were not performed like ours ( of course, there could be exceptions ), same for showers in group less common for girls, swimming suits very short like speedos for boys at that time while it was one piece swimming suit for girls at least at that time, locker rooms without curtains for boys and supervision by Pe teachers without privacy and so on ..
So I am not at all surprised to read that boys under 14 age were examined naked by a doctor and a nurse in front of their mothers and sometimes their sisters-older or younger, no matter, ( or even two sisters or female cousins) while their sisters-older or younger-were not examined naked and certainly not in their presence, they were instructed to get out of the examination room when their turn came.
But I believed that it changed when a boy reached 14 age, I did not believe that a pre-teenager over than 14 age, at 15 or 16 age for instance, could be examined stark naked and for all his examination under not only the sight of his mother which did not shock me too much but also his sister .
But I have read some relations and noticed that it could happen and not only accidentally which is something different..
I try to imagine the feeling of a boy of 16 age exposing his genitals and his butts to the view of his sister during an examination, sometimes bending over with his asshole exposed either to let the doctor have a visual examination, either for a rectal temperature which was still common, and the comments of the doctor on the development of his puberty or on his cute butts.
I guess that after it was difficult for him not be teased or mocked by his sister especiallly if she was younger, she could always recall him that she had seen him stark naked, not a pleasant situation !.