Since everyone was really supportive and helpful about my worries about how my sister and her boyfriend were raising their puppy, I thought I would give you all a little update (quick summary for those who didn't read the previous thread: they left the puppy in a car for 2 hours in the hot summer heat with no water after a 3 hour car trip, and they left the puppy alone to play with a unfamiliar bigger dog while they went out, during which time the puppy was injured and had to get surgery). I ended up having a talk with my sister alone, so her boyfriend (nicknamed: Stupid asparagus) wouldn't interfere, offering some of my hints and suggestions, and giving her my golden rule for animal caring: to always ask yourself "would you do this to your child?"
They kept the puppy for three more months, and they did improve (they invested in a large crate so the puppy could stay alone at home, read a puppy raising book, researched her breed, started caring water around with them on walks in the heat, etc...). However, in the end they decided it would be best for the puppy to find her a new home. Now she is living with some of their friends, so they still get to see her from time to time, who live in a house with a fenced yard and have experience raising dogs.
So it worked out better in the end for the puppy. My sister was heartbroken, but I respect her for making a really hard decision and being honest with herself about the situation and doing what was right for the puppy.
Thanks to all who responded again, you really helped me understand the best way to handle the situation.
"I'm in your vagina, lurkin on your garlics!" - paprikapapaya
"My vagina won't even come to the table because she's out fighting crime under the name The Pink Boom." - idatetattooedguys