Hey Lore,
I tried to get this flag by talking to the younger brother and selecting "a present from your brother," but it failed to trigger the hospital scene. I can see in the code that it's written as $shuuki_story.append("91b") whereas the one with the older brother is $shuuki_story.append("91"). My guess would be that the logic in Chapter 5 is not recognizing "91b" as a valid instance of "91" because they were defined as 2 different story events.
-Susie