How do we go about intervening with the health and welfare of our parents?

My husbands parents are 89(dad) and 84(mom) and are driving my poor husband crazy. First off they have a bulldog who is in bad health and just ruining the home with urine and poop accidents which the mom tries to clean but doesn't do very good. We have went over and cleaned the room thoroughly that the dog messes in but a week later it was back to being just as bad. The dog belongs to the nephew but has been living with them for 5 years because she can't do steps. The dad is always cold so he won't run the air conditioning and won't let the mom open any windows so you can imagine how bad the house smells and how hot it is. My husband has gotten sick over there many times and so has one of his sisters who now refuses to go back because of it. The oldest sister is an RN and the medical power of attorney but is on disability herself for mental and physical issues and isn't doing anything about it. The father won't keep himself clean and hasn't taken a shower in over a year. He washes out of a bowl of water and can barely use his arms because his rotator cuff is bad on each arm so he doesn't have much range of motion and can barely walk much less stand. My husband just got in a heated discussion with his parents last night because he told his dad that he needed to keep clean and wash at least 1 time a week to which his father replied "Ive never heard of having to do that". Then the mom starts defending the dad and my husband is being yelled at by both of them. He offered to come help and they won't let him. They turned on the air yesterday because the dog was hot and having a hard time and it did help with the smell some but it's still really bad over there. His parents have always been very strong willed but this is ridiculous. Does my husband have any rights to do anything about this? He has talked to the nephew about taking the dog but the parents won't let the dog go. His father has always favored dogs over people and it has become insane at this point. He is very confused all the time but I feel that is due to the fact that there is no fresh air ever in the house and he hardly ever leaves any more so he is just breathing in this toxic air. He doesn't feel good a lot of the time but won't take his prescribed pain meds that I have personally seen do work for him because my husband got him to take one finally one day and within 10 minutes he was talking and with it and actually sitting up at the table instead of his head on the table. My husband is at his wits end at this point. He is not getting the support of his siblings that he needs and he wants to help his parents but feels like his hands are tied. They let him mow the grass and fix stuff that they allow him to but there is a lot that needs done. What is our next step at this point? There is so much more I could tell you but it would be a book!