(((((uhhidk))))) Sorry you are feeling so low.

You said that you have talked to your mother and sister. What I read is that they are willing to monitor your illness to see if it is just stress, is that right? If it is and they are willing to monitor you I think I would find some way to explain how you really feel in another way than words - like share with them PC information on depression or another site on the Internet. By sharing they will also see that others live with depression every day.
If you do not feel comfortable with this idea I would do as the others have said and go to someone outside the family, counselor, doctor or therapist. Remember you are always welcome to post here. I wish you luck. Hugs for your day.