Do you know how long your sister is going to be living with you?
This is what I would do:
Ellie needs to go potty outside. That is not optional. You need to go back to GO with her on that, which means a tight schedule, and supervision all the time. Take her out every 2 hours, on the hour, when she is not crated, and wait for her to potty -- but before you take her out, put the monster in a crate. Every two hours, pick the demon up and put him either in a pet yard (large x-pen set up in one of the rooms with toys and such in it) or put him in a crate. Then go to your dog, snap her leash on, and say, c'mon, let's go potty. Praise her when she potties outside.
Now, about that pet yard, you can set it up in a room where people generally are so no one feels left out or closed in a bed room with the door shut, isolated. I would work with your sister to have one of the dogs in the pet yard, and one of the dogs out, maybe your dog can be out from the time you get home through dinner, and her dog can be out from after dinner until bed time -- something like that.
Try to make it fair for both dogs, though the dogs probably will have less trouble with the arrangement than the people who feel their animal is being picked on.
Good luck with this. I am sorry. Sometimes, you have to give some ground, when it comes to family. Sometimes you have to give a lot. But, sometimes they do too, and even if the sister in question never does, someone else in your family may.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.