I don't have boys - my skids and my bio are all girls.  But I see little boys that my SDs play with, and this kid doesn't sound like just a rough boy to me.  A rough kid to me is one who accidentally runs over other kids while playing, who might rush down the slide and bump into the kid still at the bottom, who throws the dodge ball a little too hard.  It's not a kid who frequently screams at or pushes down a toddler, or talks about shooting people in the face.  This sounds like a kid who needs some help.  And there is no way I would respond to orders from the skid.  On the rare occasions when my SDs do that, I tell them I only respond to children who ask nicely.  Doing what they order you to do only reinforces bad behavior that you don't like.

But as the stepmom, all you can do is point the behavior out to your husband.  You can't make him do anything to help his son or improve his son's behavior.  You can remove your child from harm's way if the shoving continues or his behavior toward her gets worse, though.  It would be a very sad commentary on your husband if he does nothing even though he knows how his son is behaving and others (you) are pointing out to him that it's not normal.

This hell spawned violent 5yo POS would not be in my home..... ever.  Particularly with my toddler there.

If you are being asked to watch him all day, then you should be allowed to discipline him. It sounds like lack of parenting is his main issue, quite frankly. If DH thinks he's just being a boy and won't back up your discipline, then refuse to watch him and BM can keep him during the day on your time. 

I would say ---find other arrangements for you'res and bm's kids, when YOU are not present in the home. TELL me who you find by this Sunday Night.  MY focus is - insert your babies name - only.

If he try's to call your bluff and he starts to walk out of the house for work, leaving you with his children again-  have a bag or two ready, grab your child and go to your Mom's or a coffee shop.

I do tend to agree with RAGS......never again in MY house and never around my baby.

Violence is never ever acceptable. Period. Dad can teach him healthy ways to let out his frustration. Abusing a toddler isn't one of them. 

Is the SS this way when his father is around? I ask because maybe he's not understanding the intensity of abuse by his child against his other child. 
 

I believe you have a very valid concern. If nothing changes I would say that I can no longer have the two of them together when it is just me with them. 

I am going through a similar situation as yours. 

Bfs daughter is 6 in a few months 7. In February I casted her from my home. Packed up all of her stuff and rid of her completely. 

She has also not seen any of the kids but one time in town just because they had literally parked right next to us since February. I got A LOT of rath from everyone from doing this, I stuck to my guns. This gal was actually physically aggressive and I drew the line at being outside and hearing the smack she landed on my 2 yr olds face. 

Do NOT put up with that, just don't! This would be a good time for your Mama Bear claws to come out. For your kids especially but for yourself as well. Speaking of Mama Bear, when you see someone intentionally hurt your child it takes un-godly amounts of strengths not to launch the perpetrating child across the room.. and than what, your in jail for child abuse. - Don't do this to yourself. 

Don't allow for them to come back until the situations have been addressed and fixed. Period.