Sentence 1: My niece bought as guinea pig for her ten year old daughter. 
 Sentence 3: She thought they were both females 
 Sentence 4:  She was mistaken, and a litter was born 
 Sentence 5:  She was able to sell the babies to a pet store 
 Option 1: She bought one male that had been neutered. 
 Option 2: She bought another one for company.