Dad doesn't care whether he sees her or not. Slightly confused at times. He's almost 96. She brings her "therapy" dog that dad's dog hates and tries to kill every time. She has her "friend" drive her down and ends up accusing him of beating her, and I end up putting her up in a hotel because I can't stand her around my house. I will NOT allow her dog in my house except the shut enclosed mud room. She smokes. She had scabies. She has a plethora of health issues and miserable life brought on by her own decisions, and I have had very little to do with her over the last 3 decades, but have been trying very hard to accommodate and over look many things since I have always felt UP TO NOW that she deserves to see her father.
Do I have to let her in my house?
No, you do not have to let her in your own house if your father doesn't care to see her.
If this is a health issue you certainly don't have to.
If her visit includes a dog that is a danger to the household you don't have to allow her in the house.
You may have to get a stay-away order if things progress.
Tell her that you will alllow her to visit Dad by phone.
If you believe that there is at all a way to continue as you have been, at least twice a year, I would do that to avoid all the hassle and issues that will come of forbidding her in your house; only you can tell if this is possible for you; you sound as though you have tried and are at the end of your rope.
As your father doesn’t mind either way, I would just tell your sister that sorry, “This is a bad time”.