What is the significance of the narrator decorating her room with "fancy" things and attempting to separate her space from her brothers
The story Boys and Girls is about a young girl who refuses to accept that boys have a certain role in life and girls have a very different role. She refuses to do kitchen work and would rather feed the animals and do work around the farm that society deemed "men's work" at the time. She fought against the gender roles society forced on her until she finally gives in and accepts her role in society.
When she changes the room, this signifies that she has given up fighting and accepted the role society thrust upon women of those times. She adds the "fancy" items in her room and wants to be away from her brother, because she has accepted that they are different and she doesn't want to feel like he is watching her in her room anymore. Even her dreams change from being the hero to being the princess who needs to be saved. So the changes to her room signify the fact that she has officially fallen into the role her mother and grandmother wanted for her.
For more information check out the link.