Has anything happened recently that may make her afraid to sleep alone how long has she been like this ? What about school is all well there everyone get on okay ? Have you spoken to her about it and asked her why she is afraid , maybe talking it through would help.
Why not let her sleep in the same room as her younger brother. Usually when my children reached puberty I did not let boys and girls sleep in the same room, but I might make an exception in this case.
Meanwhile try to find out if she has had any bad experiences lately. Is it possible that when sleeping in her room alone that someone entered and molested her? Try to get her to talk (without leading prompts).