I'm not sure I have a sound answer for this because I am also dealing with a family member who is living is a dirty, smelly, extremely hoarded house. My sister will take on stray cats to "save" them. This probably stems from a divorce and a mother who "wasn't there" for us growing up. My mother was there, but never participated in any of our activities and such. With that being said, I too had to read and learn about how to deal with this situation.
Durning this past Holidays (Dec 2011), I walked in to my sisters home and almost threw up from the smell and air quality. There was a small path from the front door in the kitchen, with both sides of the sink piled high with dirty dishes and food... I went no further into the house and didn't say anything judgemental or negative. I stayed for 5 minutes... I told her I loved her but was hurt that she chose to live like this. I told her I would help her if requested and put her in touch with specialists if SHE requested. Remember, this is all on their terms. You can't force help on anyone if they don't realize they have a problem. I am there for her, but will not be visiting anytime soon unless she reaches out. Some people may find this as a terrible statement, but until you've seen and experienced a situation like this... you can not judge me.