I think you need to do
Public static List<string>() sintomas = new List<string>();
Then in your other class:
private YourClass Yourname = new YourClass();
Then you could acces the list like this:
Yourname.sintomas.add("yay");
Update
On the start of your class so after:
public partial class AddisHome : Window
{
You need to create your list like this:
Public List<string> sintomas = new List<string>();
It's public so other classes can acces it
Then on your second class:
again on the top of your other class after:
public partial class Personaldata : Window
{
You need to add this:
private AddisHome yourvar = new AddisHome();
then inside your function you can call:
yourvar.sintomas.add("yay");