Keeping a dog inside the house

Assalamou ailekom I would like to know if my prayers are accepted or not if I have a dog in the my house. My brother adopted it without permission from us but we had to keep it since we got emotionally attached to it. I know that angels don't enter the houses of people who keep their dogs in their houses and that's dogs are just permissible if they are guard dogs. We see it as a pet but it can serve as a guard dog too maybe because when guests or strangers are comming to our house the dog goes to window so we know that someone is coming. So I'm a little confused if it's a sin or not in our case to keep the dog. Thank you in advance.