First of all, do you live with your sister and are you prepared for
this to be your dog if she is not capable/willing to care for it?
It's usually a VERY bad idea to purchase a pet for somebody else, and
most good breeders won't let a puppy go unless they know the person
who will actually be raising it.

Second, Las Vegas is way too hot for any dog to stay outdoors for any
length of time. A Saint Bernard is out of the question, to my mind --
I've seen people at shows trying anxiously to cool their dogs down
when it's not even 75 degrees in the room.

As for a Shepherd, the shepherd people can answer better, but I
wouldn't want you to get ANY dog that would be outside in the summer
in Las Vegas.

kassa