I agree the help line is really good and I am sure they will help sort this all out in your mind. I would guess that they want to see if the letrozole can shrink it and then decide about removal of the lump. Your sister certainly isn't terminal and as you say if it is a new primary lump then they will still be trying to cure it. I would have thought even if its a secondary of the same lump and confined to breast tissue then they will still be hoping to eradicate it. Whether its primary or secondary I am sure they will be treating it with all they have and the prognosis even with a secondary lump can be very good and getting better all the time. Its an awful shock for you all and will take time to get your head around it all. Just know that sisters mean the world, I wouldn't have got through my BC without my sister, she was my rock all the way through.
Just keep positive, all can be fine in the end and there is loads I am sure they can do - lots of love to you, let us know how she gets on.