My sister, who is 25 years old, sleeps way to much. (about 14 hrs a day if given the opp) always complaining she is tired- etc. She has been checked for mono, low iron, blood sugar, has a history of a thyroid disorder, but last time she went to the doctor

Many causes. Lack of quality sleep has many causes and conditions that may be responsible. An overnight psg sleep test may help to determine if there are sleep disordered sleep conditions that are producing this lack of refreshing, quality sleep that can cause daytime tiredness. See a sleep specialist for further advise and recommendations.
Your . Your sister is lucky to have you, and you are right to be concerned. Some people do need more sleep, but that is a concerning amount to sleep every day. She may want to see a sleep specialist, because although she is sleeping so much, it may not be "good sleep" due to sleep apnea or another condition. In addition, your thoughts about possible depression affecting sleep are correct, and she should have an evaluation by her doctor if she will agree to this. If she is on any medications, they should also be evaluated as some medications can make you too sleepy.