I do not know how you could stand that.

If it was anyone in my family doing that (thank GOD no one in my family ever, ever abused/neglected a dog), I would first give them a long long talking to, if that didn't work I would resort to yelling (and maybe giving them a good kick), and I would take the animal if that did not get through. Seriously, I know it's family, but if I were in your situation, I would not even want to consider people with that behavior as my family... especially if you've talked to them about it repeatedly. You need to at least call the local humane society, if they've killed animals in the past.