This is kinda long and it isn't my first language so I'm sorry for the mistakes.
Me (F21) and my family live in the same ground* but n separate houses with my aunt and my cousin (M20) we don't get along well because my aunt likes to blame my father for everything since they were kids..., they have to move away because their house isn't safe (because of the last earthquake in my country), so they choose to leave alone their dog on their house with an awful made "fence" (that was useless because the dog could scape anyways) they have left about 15 days ago and we bath and feed the dog because they come to feed it one time every two days or so. The poor dog kept barking all the first five days.
On Friday when my brother (M25) is leaving to university the dog run out as fast as it can and my brother gets stuck on the door because of his backpack. He searched for it for ten minutes and come back to tell my mom, he leaves to college because he was having a important meeting with his thesis assessor (?) And my mom took her car and went to look for the dog. Nothing.
We told them about that. "Your dog escape again"
At the 7pm I was alone with my brother.
My cousin enters in the house without asking first and ask my brother about his dog in a very agressive way, as if taking care of him was my brothers obligation. Cousin told him that he doesn't give a fuck about his college, that he is going to help him found his dog, I repeat, the dog that they weren't taking care of... My bro is a very calm person so he just nodds. My cousin leave and slaps the door.
I forgot to say that for years ago they let one of our dogs (a puppy) escape and they didn't tell us, I found it dead on an avenue, a car killed it.
We didn't complain. It was an accident...
So now, my cousin is full of anger and wants to "talk" with my brother because "they made a deal"... He even blame us because my aunt have a facial paralysis after the problem of the dog.
Almost tried to threaten us "I don't know if something is going to happen or not..."
So, we search for the dog the first day (including me when I came back to school) but because of his attitude we don't want to do anything. We're just waiting the exact moment to put a demand on him if he hits my brother or if he kill our two dogs.

So, what do you think?
Are we the assholes? :( 
I'm very scared because I'm short (153cm) and I can't protect my brother(176cm) because my cousin have the height of a monster (2m) and I don't want my dogs being poisoned...