The given sentence "She decided to take her brother's key because she wasn't sure when she would be back." is a compound-complex sentence consisting of an independent clause and a dependent clause.
The given sentence "She decided to take her brother's key because she wasn't sure when she would be back." is a compound-complex sentence.
Compound-complex sentences consist of two or more independent clauses and one or more dependent clauses.
In this sentence, the independent clause is "She decided to take her brother's key", while the dependent clause is "because she wasn't sure when she would be back".