it is "kids being kids." My brothers regularly beat the crap out of each other. My nephews are constantly rough housing, wrestling, jumping on each other etc. My grandson (who is only 10 months younger than my youngest nephew) is usually right in the mix. My niece joins in as well. My kids did it. And yes, it went too far at times and it does go too far at times. But it is also a relatively normal part of childhood. I am probably the only kid in the family who was not a fan of rough play. I was usually reading a book. 

My family allows rough play. If it goes too far the kids are disciplined but we do allow it. Yours does not and that is also okay. I think a discussion with SS about the differences in the households and possibly an outlet for some of that pent up rough play would be good. Football, karate, soccer. Something super active and a touch competitive. 

In my opinion it is not 'boys being boys.'

I have 2 sons. Their age gap is 3 years (YBS is 13 and OBS will turn 16 in Nov). They do not hurt each other and never have purposely. They have shot at each other with Nerf guns. They've wrestled. But, they always help the other up. 

SD was always the mean one because she was the only child for the first 5 years of her life. She would pinch the bottoms of oldest' s feet to make him scream as an infant. She would steal and break his toys. What stopped it was when she twisted oldest's arm and gave him a Bible cyst and bit his face. I called the sheriff's office out on that sh*t.

You have a major problem in your home and you are not over reacting.

Pretty sure SS dislocated your son's arm too and that was an ER trip. 

SS is not to be trusted!

I spent my childhood being punched, slapped, kicked and bullied by my highly strung, violent older sister. The abuse only stopped when I hit puberty and grew taller and strong enough to beat the snot out of her. It impacted my self esteem and my ability to trust, and we have never had a normal sibling relationship.

That said, I know families where the kids would never dream of raising a hand against a sibling, but these are households where the kids have been taught to articulate feelings and frustrations and have healthy coping skills. At seven, your SS is old enough to grasp that there are different rules and expectations in his parents' homes. Your DH did a good job of handling the situation, but going forward you might want to work with your SS to help him learn healthy ways to manage his anger and aggression. I like the suggestion of involving him in sports, too. What he's allowed to do at BM's should not be allowed to impact your household. Roughhousing is one thing, but hitting/punching is quite different and shouldn't be tolerated IMO.

At this point, no, I don't believe "there is something wrong" with your SS7 in that a good *ss whipping by another kid bigger than he is won't cure. 

But that doesn't mean you can trust him not to currently really hurt the younger smaller child. IMO, consequences probably need to be a bit stronger than being sent to bed early. Where I have found males will be quicker to come to whopping on each other more than females, females will go to screaming in your face argument cat fights (even with kitty claws out).

But it can not be tolerated when a much larger child starts physically beating on a much younger child who is also too, much smaller. Your younger one is still a babe, he can't think as fast and well, mean, as the older child can, Thus the little one is taking the brunt of the fight.... and he could indeed be really hurt physically. Right now, for SS to be whopping on the toddler, it's a kin to if Dad were to whop on SS7. Something of course would not turn out well for SS7. 

You can't control what BM allows in her home, but you and Dad certainly can control what goes on in your home. The kid is being allowed to be a big bully at BM's (partly due to his age and size). But yes, he needs to know your home will have zero tolerance at this point .... and Dad may have to be pretty stern and put a bit of fear of the devil in SS7 to keep normal healthy sibling rivalry on a 'fair' level. 

My oldest son and my oldest daughter are my two kids closest in age. Four years apart. But size wise , well he is now well over six foot and the daughter is under five foot. My oldest daughter is very petite. My oldest son was like  major league football player compared to her. He wasn't allowed to bet on her. But she really knew how to pull his chains. He'd take just so much from her before he'd simply scoop her up over his shoulder and put her down out on the deck. Lock the door and let her scream "Mmmmoooommmm, he tossed me out". Yeah, after about ten minutes I'd make him unlock the door and let her in. I can't imagine what my neighbors who heard her whining and b*tching thought. 

It would be 'easier' if your home had 100%, but that's not going to happen. So Dad is going to really have to keep an eye on this and come down hard when the boys (or specifically the older larger boy) gets to rough. And when Dad isn't around you have to be able to take charge and enforce.

And did I read not long ago there is a third little bundle coming your home's way? I think so, but may have members mixed up. If so, now is absolutely the time to get  grip on the rough housing and bullying. Next up could very well be your now youngest son taking out his new learned tactics (taught by oldest brother) on baby brother. 

Supervise.Intervene. Consequences grow larger and more fitting as new incidents occur.

Plop them both down at the kitchen table, with neither being able to leave the table until they 'work' out their disagreement. My oldest son and daughter hated that, but after about 30 minutes, they would be sitting there laughing and buddies again. You and Dad will find what works and what doesn't for your home. 

And that's leading your 7 year old to beat on your BS.  My guess would be that he needs some therapy with a decent counselor and/or family therapy for the group.  That doesn't mean that there's something "wrong" with your SS, it's just that kids at that age don't usually have the ability to express their feelings in a constructive way, and resentment is a tough one to explain, even as an adult, so they go physical.

It makes me thing of my skids.  SD13 beats the crap out of SS almost 9 at any chance she gets.  He is BM's golden child and does no wrong, while SD is criticized and belittled as much and as often as possible.  And SS knows it and exploits it, getting SD in trouble on the regular.  Neither child feels secure in their roles with BM, so they're in constant competition, which builds resentment and results in fighting.  SD is bigger than SS, so that means she wins, but he starts with her regularly, and he's very mouthy.  DD12 has been instructed to stay far away from all of it and she largely keeps to herself.

We're working on the theory that it all evens out in the end.  Both skids are in therapy because of the insecurity of BM, so whatever.  That seems callous, but that's how we have to "function."

Eh, it's hard to judge without being there. BS5(nearly6) and BS3 are really rough and tumble boys. They routinely jump on each other. I've seen things like you describe and when I intervene both boys are laughing. I've also seen when it goes too far and one boy (or both) is crying. There are consequences for hurting each other/going too far and they both know those consequences. They take their punishments and then go right back to playing with each other, wrestling, and jumping/climbing. I feel like it's a parental judgement call to figure out if it's playtime or meanness. 

As an example, Christmas morning last year I had both younger boys dressed nice for church with Santa hats in front of the tree. I wanted a Christmas picture, so I asked the boys to hug each other. As I held up the camera both boys punched each other in the arm, and immediately devolved into giggles. You bet I sent that series of photos out titled as 'Brothers at Christmas : So much LOVE'

Later in the day, BS3 wanted BS5's toy car, BS5 wasn't sharing, so BS3 tackled BS5 and tried to steal the car. No giggles, no smiles. That wasn't play, and BS3 recieved his consequences.

to never let BS out of line of sight when SS is there. 

I’m sorry to break the news there is no timeline for this. 

I had to do this until SDs aged out of visitation. 21 and 18. 

YOU, not your DH since clearly he is incapable need to take on that responsibility. Wherever you go in your house, you need the bring BS along. 

I think SS is a candidate for residential placement. What kind of advice have you sought for handling his dangerously aggressive behaviors? What kind of treatment is he getting?