I agree - all you can do is tell her that what she is wanting to do is NOT the best way to buy a puppy. And I would NEVER EVER buy online without seeing the puppy first (and seeing what it's living conditions are) - or at least KNOWING who your buying from. But I certainly would not UNDER ANY CIRCUMSTANCES buy from a web site that is selling many different breeds of dogs - that just SCREAMS out
PUPPY MILL.
Does your sister know about all the YEARS of heartache that can come with these Puppy Mill dogs? All the health problems which can lead to super high and very costly vet bills?? Not to mention sadness for HER if anything happens to the dog (it gets very sick, or even possibly dies).
I don't know which web site she is wanting to buy from - but when you mentioned that they sell many different types of dogs - that was a HUGE red flag for me.
All you can do is warn her - but it is going to be her choice in the end. I just hope you don't end up having to tell her,
"I told you so"