I have two relatives who have exhibited animal hoarding, although there aren't any dead ones stashed anywhere. It's been really sad .. both are elderly women living alone. One has children, the other doesn't, and the children have tried talking, cleaning, etc.
I'm sorry for your relatives. And it's tough for family members to get the relatives to someone who can help.
A neighbor came very close to this situation. At one point, she had something like 40 cats. I believe someone got her to seek some help before it all got out of hand. I know she still goes for some sort of counseling or therapy. But she's now down to 8 cats (which she can manage since she has a large house and the financial means)...the rest I believe were sent to shelters. She does from time to time trap some wild ones, take them to a vet and have them spayed/neutered, but she doesn't keep them. At least now, she doesn't have a house full of cats.
Your relatives' loneliness may be something contributing to this. Seems to me that most hoarders are alone. Like I said, I don't know if anyone knows for sure what causes this...perhaps there are a myriad of causes...who knows?