19 year-old nephew beat up my 12 year-old son after an argument. How should this affect my relationship with my sister?

Update: My sister and I both have 3 children, and our families visit frequently. We have a great relationship. Unfortunately, she has a short temper and can be verbally abusive when upset, traits which she passed onto her eldest son. "Ben" is 19, and seems fine at face value. He got good grades in high school,... show more My sister and I both have 3 children, and our families visit frequently. We have a great relationship. Unfortunately, she has a short temper and can be verbally abusive when upset, traits which she passed onto her eldest son.

"Ben" is 19, and seems fine at face value. He got good grades in high school, has a part-time job, and is polite to adults. However, there are deeper issues. Ben has a poor relationship with his mother, since he dropped out of college due to a severe video game addiction.
Update 2: This summer my sister's family was visiting us. Ben spends 12-15 hours every day playing video games on our Apple computer. His obsession is so strong, he will often force the younger kids---all elementary/middle school aged---to get off the computer whenever he wants to use it. 99% of the time, the younger... show more This summer my sister's family was visiting us. Ben spends 12-15 hours every day playing video games on our Apple computer. His obsession is so strong, he will often force the younger kids---all elementary/middle school aged---to get off the computer whenever he wants to use it. 99% of the time, the younger kids concede because they are intimidated

My son "Robert" (age 12) is different. He has a strong sense of fairness, and is more likely to speak up.
Update 3: Last week, when Ben demanded that he get off the computer, Robert did not move, and instead replied that he was going to finish what he was doing first. This made Ben angry. Robert kept insisting that, "It's not fair that you get to use the computer all day, when everyone else barely gets to touch it at... show more Last week, when Ben demanded that he get off the computer, Robert did not move, and instead replied that he was going to finish what he was doing first. This made Ben angry.

Robert kept insisting that, "It's not fair that you get to use the computer all day, when everyone else barely gets to touch it at all," and "This is not your personal computer. I have just as much right to use it."
Update 4: Ben kept saying, "I don't care. I'm not your grandma. It's not my responsibility to care about your feelings or your rights. I'm not going to compromise with you." I intervened when Ben raised his fists and threatened to beat up Robert. It happened again a few days later, when my sister... show more Ben kept saying, "I don't care. I'm not your grandma. It's not my responsibility to care about your feelings or your rights. I'm not going to compromise with you." I intervened when Ben raised his fists and threatened to beat up Robert.

It happened again a few days later, when my sister and I were out shopping. This time, Ben lifted Robert out of the chair and tossed him onto the floor. The boys argued for several minutes had a shoving match, which Ben easily won since he weighs twice as much.
Update 5: Robert grabbed the computer mouse and threatened to throw it in the toilet. Ben slapped him several times across the head and neck...before pinning Robert onto the computer table and whacking him very hard across the buttocks 20x with a wooden coat hanger, while lecturing him on "how naive and foolish it is to... show more Robert grabbed the computer mouse and threatened to throw it in the toilet. Ben slapped him several times across the head and neck...before pinning Robert onto the computer table and whacking him very hard across the buttocks 20x with a wooden coat hanger, while lecturing him on "how naive and foolish it is to pick a fight that you have no chance of winning." Robert was eventually crying, and the "spanking" left welts and bruises all over his bottom.

In light of what happened, I don't see how
Update 6: how our families can continue having a normal relationship. The kids have always had so much fun playing together, and while Ben always been somewhat domineering with the younger ones (a trait his mother also exhibited in her youth), I am in disbelief that he possessed such inner darkness. I am shocked, hurt, and... show more how our families can continue having a normal relationship. The kids have always had so much fun playing together, and while Ben always been somewhat domineering with the younger ones (a trait his mother also exhibited in her youth), I am in disbelief that he possessed such inner darkness. I am shocked, hurt, and disillusioned at the TOXIC and emotionally imbalanced young man that my nephew has become.

How should we move forward? Can I even continue speaking to my sister?
Update 7: In retrospect, as I examine the past few years in a careful and honest manner---I have to confess that there were signs all along that the relationship between Ben and the 5 younger kids has never been a truly healthy one. Ben has truly adopted his mother's penchant for being verbally abusive when stressed... show more In retrospect, as I examine the past few years in a careful and honest manner---I have to confess that there were signs all along that the relationship between Ben and the 5 younger kids has never been a truly healthy one.

Ben has truly adopted his mother's penchant for being verbally abusive when stressed out or frustrated. I recall many occasion when he would call the younger kids "idiots" or "retards" or "weaklings" when he felt that they were incompetent in various games or other tasks
Update 8: My sister also has an 11 year-old girl and an 8 year-old boy. On our side, Robert has a 13 year-old sister and a 10 year-old brother. In the past, the younger kids have always looked up to Ben and greatly admired him, since they felt he was super cool. However, there was always an underlying element of fear. I... show more My sister also has an 11 year-old girl and an 8 year-old boy. On our side, Robert has a 13 year-old sister and a 10 year-old brother. In the past, the younger kids have always looked up to Ben and greatly admired him, since they felt he was super cool.

However, there was always an underlying element of fear. I could always sense that the younger kids constantly felt that their oldest cousin/brother's approval and validation were prizes to be won. These days, things have changed as the younger
Update 9: kids have grown up and started realizing that it cheapens and diminishes their own self-worth to idolize an older cousin/sibling. This summer might be one of the first times ever that one of the younger kids actually stood up to Ben and spoke to him like an equal. Robert told me later that he would do the same... show more kids have grown up and started realizing that it cheapens and diminishes their own self-worth to idolize an older cousin/sibling. This summer might be one of the first times ever that one of the younger kids actually stood up to Ben and spoke to him like an equal. Robert told me later that he would do the same thing the next time Ben tried to kick him off the computer, because "He can't keep pushing me around forever."
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