My Boyfriend's Brother Tortured and Killed Our Cat

I live with my boyfriend and his family:  His mother, father, older brother, and younger sister and brother (twins.)  The twins are 11 years old, and the boy is autistic.  That's all I know about his diagnosis so don't ask. 

Neither of the two kids are in school.  They eat whatever they want.  The sleep whenever they want.  Did I mention the whole family (mom, dad, kids) all sleep on the living room sofa?  Not one person cleans up after themselves.  My boyfriend and I are constantly cleaning up after these people.  The two kids are seriously mistreated.

The boy has had no therapy.  He's allowed to walk around naked throughout the house.  He punches and kicks his mother.  He places foreign objects in the microwave; empties out all the liquid from any bottle he can find; he doesn't know how to wipe himself after using the bathroom; he wipes his blood and excrement on hand towels and mirrors;  he never wears pants;  he is allowed to use the computer whenever and however he wants, with no supervision.  He has flooded the house on numerous occasions, and my boyfriend and I are left to clean it up while his parents sleep.

Meanwhile the girl is constantly exposed to her brother's genitalia, has to feed him, has to make sure he doesn't get into any trouble, and has to dress him.  She is his twin, 11 years old.  WTF.  She has to take care of her brother who is twice her size, and can't even form one correct sentence.

The other day I went downstairs to grab the mail.  My bf's mom was in the den, the younger girl was sleeping on the couch and the boy was in the bathroom.  Whatever.  Except that when I went past the bathroom, the carpet was soaked.  Like, in puddles.  So I quick grabbed their mom and she broke into the bathroom that he was flooding.  Then I heard the worst thing possible come from inside the room:

"I got Stinker died."  Then cackling, maniacal laughter.  Rinse and repeat.  Over and over again.

He killed one of our three cats.  Stinker was about 20 years old, and the sweetest cat around.  He doesn't even have claws to defend himself.  Upon further investigation, my boyfriend found that Stinker was fed shampoo, and shoved into the toilet, the boy's hand around his throat, and then shoved into the toilet's drain with the plunger.  Stinker's eyes were still open when we recovered his body.

This is the second cat the boy has killed.  The other one he doused with water and covered in litter then beaten and strangled to death.  But his parents were out of town, and refused to believe he actually killed this cat, Gia.  They said that the cat was all ready dead and he was just trying to wake her up.  BUT THEY WEREN'T EVEN IN THE SAME COUNTRY.

The boy also threw my cat out of a two story window when she was about 3 months old. She survived, none the wiser, but still.

All three of these instances have seen no consequence.  It's as if they never even fucking happened.  My boyfriend's parents still refuse to actually get his younger brother help.  They won't even put him in this state of the art public school specifically for children on the spectrum.  WHAT THE FUCK is wrong with these people?

This boy has killed 2 cats, attacked another cat, set a parrot free.  I'm afraid of one day waking up to him over me with a knife.  I'm afraid that he will soon take out his aggression on his younger sister:  there are things worse than death, you know.

What should I do?  I'm at my wit's end with these people and I really think some sort of authority should be brought in at this point.  I really, really need help BEFORE something even more terrible happens.