I'm going to tell my sister to let out the dog
Why do you need to tell her to let the dog outside?
He smells and I don't want to have to smell him all day.
You should be able to let out the dog yourself, if you really want him out. Your sister will think you're lazy if you demand it, so you should either do it yourself or ask her nicely.
I'm not lazy and I'm kind of too busy to let him out. I have a real job, so I don't have time to let out the dog. Just because I don't have time doesn't mean my sister is responsible for it.
The dog belongs to your household and you equally share the responsibilities to take care of them not just your sister. If you are the one bothered by his smell, then you can bathe him and let him dry, as it can be cold outside and he may get sick.