How would I write:

To the X Family,
I'm deeply sorry for your loss.

Thanks! (Don't have to translate that one!)

Para Familia X (in Portuguese we don't use X Family but Family X )
Sinto muito pela sua perda.

hope I helped

You did help, thank you!

The above is certainly correct, however, I would like to offer my addition.

I have used this one, sounds a little more eloquent.

Gostaria de extender minhas condolencias a voce e sua familia.

M.

Muitas condolencias para voce e a sua famila.