Take another look at the sentence:
That she couldn't sink a basketball is a subordinate clause. A subordinate clause cannot stand alone as a complete sentence. Because she didn't have expensive athletic shoes is a second subordinate clause. Two subordinate clauses can follow each other without punctuation.
For future reference, you might want to remember that 99 percent of the time because does not require a comma before or after it. Using a comma with because is a common error.