These are small kids. Do not give them the power to decide anything. They aren't equipped for it. For now, SD isn't to be around the younger kids unsupervised. If caught doing hurtful things to younger kids, an age-appropriate punishment (time out.) If "caught" being nice to the younger kids, she gets praise for what a good big sister she is, how nice she is, etc. You and your DH direct this. SD and BM have zero input on what goes on in your house and whether or not SD wants to come over is irrelevant. When your younger kids are old enough to tell you if someone is hurting them, they can be left alone to play. You guys can get a handle on this now. The kids are still young enough. 

BM is wrong.  it's not up to her FIVE year old whether she wants visitation.. it is up to her FATHER.

How are the kids separated in your home.. do they have their own rooms.. are they sharing?  

I don't think that just brushing it off is the right thing.. but maybe DAD needs to be around and parenting his child instead of you having to wrangle 3 very young kids on your own?

I do think that dad needs to work with his child on this.. and I think he should exercise visitation with his child. .but he should be there to make sure no shenanigans happen.. if that means that the kids are separated at night when sleeping etc.. that needs to happen.  the kids shouldn't be sharing a room until this is more sorted out.

sibling rivalry.. including fighting and othe rjealousies is not abnormal.. but it is not acceptable and the kid needs to learn a quick and fast lesson that it is NOT acceptable behavior in your home. 

I personally wouldn't leave them alone. Ever. I would never trust that 5 year old around one on one. BM (nor her dad) is raising her properly to have boundaries or respect for others including her own siblings. 

You a good one. I would have cursed that bio Hag the moment she refused an apology then insinuated your kid wqs the problem. Do not trust that woman nor her violent kid.  Of course she will send her mini-domestic t3rrorist over so she can get her kid-free weekends. 

Sibling rivalry is normal to an extent however step sibling rivalry can cause more strife as there are more adults involved who may not see your perspective/may refuse to handle the situation appropriately because their precious angels can do no wrong sends them deep in denial. 
 

For this reason if I were you I'd be more stern regarding importing kids who act out violently, abusively, etc - unless there is a CLEAR understanding with bio parents and their kid - that violence/abuse in the home will not be tolerated.

That 5 year old does not have to be in your home especially if she is a danger to your younger kids. Do not give her any more opportunities to use those young kids as her emotional punching bags. Let Her dad know your stance = if she put her hands on those kids again then he can do his overnight visitation in a Air B n B or hotel. 
 

Sounds like you have an inkling something is off about the 5 year old which prompted you to record. So keep recording when the 5 yo comes over. You may need to hire a lawyer and start reporting this behavior in case your husband acts willfully obtuse then you have professionals in your corner advocating for your peace and your younger kid's safety. 
 

Your SO needs to be able to parent/supervise his daughter when she is visiting. If he is unable to uphold his visitation, then so be it. She doesn't need to be at your house if Bio Dad is off somewhere else.

What's your living situation with SO like?

He can not leave his 5yo alone for a minute.  I would get cameras for the living room. To see first DH actions then SD actions  to see if both are at fault. DH into big happy family. Brother and sisters all loving each other.  BM is out of this this is your home BM has no say in the control of your home.  You can not have a t yo and 2 yo together when the 5 yo is the way she is.  Understandable, SD is starting to realize that your kids have a different life then her,  they are not going between two homes. You kids have a BM and BF living with them. They get to do things when SD at BM.   She knows you do more for your bio kids then her.  You must get a bigger place. Three kids and one bedroom. Doesn't work.  Can't have a baby with a 6 yo. Expecally that that 6yo. I could not sleep at night with them together, there needs to be a wat to keep a camber to make sure SD doesn't do anything.   Your problem is DH is not going to go alone with this.

Stop giving BM any presence of standing in YOUR family. She has no place, input, or value in your family.

Stop talking to the 5yo other than to be very firm with her that this will never be allowed. Then, swat her on the butt, march her to an isolated corner by a twisted ear, plant her nose in that corner, and she can stand there all day except for supervised meals, toilet breaks, and bed time until she goes back to her idiot BM's place.

Why she did what she did does not matter, that she did it matters.  Stop giving a 5yo any decision making authority in your home. She comes on the COd schedule or daddy spanks BM immediately with a contempt motion for interfering in his COd visitation.  See how BM likes getting her ass bared in court and see how SD reacts as she sees mommy get consequences for interfering in visitation.

And... WEB Cams.  Everywhere other than bathrooms.  A 5yo who is this toxicially manipulative needs to have a baby monitor in her room. As should her younger sisters.  I would not put it past this kid to do a late night visit to the baby's and toddler's to try to punish them for the 5yo's jelousy.

Trying to convince a toddler to bite themselves and trying to convince them that they like being shoved, oh hell no.

She probably is jeally because your kiddos have more of dad and mom than she does.

Nanny cams definitely, seems you got that.

Its extremely common for children to "play the houses". Tell BM one thing, tell BF another. 

I used to tell Powersulk SD17 that when she got big enough and mad enough, she would defend herself to the extent her sister would have some fear. This when SDthen16 Feral Forger was beating up on her 8 yo sister. And she did. Slammed a shoe on her hand and she had a sprained fingy, poor pwincess.

So, I kind of like the approach of teaching kiddos that defending themselves is good, ezpecially since you will have cameras in case 5 yo violent bratola will more than likely make accusations...

One thing I will point out is that your husband is the child's father.. and while notifying BM that there has  been an issue is fine.. HE is the one responsible for his child and his child's behavior in his home... and he needs to get on top of this.. himself.. 

#1 I'm shocked that people are asserting that your DH shouldn't be able to shower while his child is visiting. That is just crazy to me. This doesn't mean that you have to supervise her while he's in the shower. A typical 5 year old should be able to play unsupervised for the amount of time a shower takes. Just make sure she stays in the house and isn't left alone with the little ones.

#2 I will echo what ESMOD said - this isn't fully BM's responsibility. Your DH is one half of SD's parents, and he is just as responsible for teaching her to behave as BM is. He needs to get a handle on it and not just expect BM to magically correct/control SD's behavior when she isn't present.

#3 Sending a child away when they misbehave is INCREDIBLY damaging. It won't take long for her to internalize that if she isn't perfect at all times, she will be abandoned. That sets her up for a lifetime of relationship problems. 5 years old is not too late to intervene with therapy or even just discipline at home. Of course you have to make sure your children are safe, but isolating a 5 year old is not the way to go about that. It's heartbreaking to me to think of a 5 year old being separated from her family because she hasn't been properly equipped to handle/express her emotions.