11yr.old half brother take a knife after his brother

My step sons half brother took a butcher knife after him and said I want to kill you.His mom did nothing but tell him to settle down.Every thing the half brother does my step son gets in trouble for it even though the mom see the half brother do it.This 11 yr.old hits himself and makes red marks and tells his mom that my step son hit him.The 11 yr. old got mad at my step son and ran into the house and broke 5 plates on the floor.When we gave my step son gifts for Christmas his mom took the clothes and gave them to the half brother and made my step son wear jeans 2 sizes too big.When he came to out house for his once a month visitation he was wearing a USB code to hold up his jeans.We gave my step son gifts for his birthday and the half brother broke them.My step son told his mom he wanted his half brother to buy him a new one and the mom got mad and said no he don`t have to buy you anything.His mom does nothing but yell and discipline my step son all the time.His mom and her boy friend leave my step son and step daughter who are 13 and 14 yrs. old to watch the half brother when they go out at night and I am afraid that this 11 yr. old is going to hurt them.What should we do they live in Texas and we live in Oklahoma.We tried to stop the My husbands ex-wife from moving with the children but the courts let them move.