Sure, here are the sentences with the appropriate articles filled in:

1. Do you know that **the** Andes extend through seven South American countries?
2. **Ø** doctors are giving us **Ø** advice on how to avoid **Ø** stress.
3. There is **a** special training program for **Ø** employees working from **Ø** home.
4. I have two sisters. **The** older one is **a** nurse, and **the** younger one is **an** athlete.

I hope this helps!