Yes, many of us on the site have left partners, or on the brink of it.  If you decide to stay with your partner - do NOT let BM enter your home - get the locks changed immediately!  I would also advise you to never be alone with your SS if he is prone to making up sexual abuse allegations.  DH and his ex may have no boundaries, but it doesn't mean you can't have them! 

It's your life to live.  You can not have a SC running your life.  You can be afraid of SC telling lies about you. And ruin your reputation.  If DH , can't parent him and doesn't get him the help he needs. You. Have to leave.  What are you getting out of this,  at least he should get SS to a place for treatment. Get it on record he's nuts. 

You might have to live separately until they age out of the court order.  
 

You do have to protect yourself and make it clear to all that the marriage comes first, then any responsibility you each have.  Marriages are priorities,  skids are responsibilities.  There's a distinct difference between the two.  His baggage should never put you in harms way, aggravate you, be a financial drain or upset your peace in your home.  Any sane adult should expect this to happen and dysfunctional unhealthy people trample over those logical normal boundaries.  

Your SC lying about being assaulted is a deal-breaker for me. I used to work in LE types of jobs and I couldn't afford to have my reputation and career that I worked so hard for destroyed by a lying brat. 

Along with a kid that has proven to lie, you have a weak parent. That combination is disasterous. 

As long as you stay in the same home with your SO and his lying brat, you are at risk for not only losing your career and reputation, but if you go to jail, even on remand, waiting for investigation, you will be shit-kicked by other inmates. I used to work in the system. There is no proving yourself should you end up in the slammer. I've also seen people's lives destroyed even after complainants have confessed to lying. You never recover from it. It's not worth it. If you really can't tear yourself away from your SO, then at the very least, move out and date him when he doesn't have his kid. Meet for dates outside of the home and nowhere near the kid. 

Do you have your own bios? If so, all the more reason to leave the home. You stand to lose them should your SC lie to CPS. You will lose them pending investigation at the very least. I worked in that field too and I've seen it. 

Get out and get out now.

I'm not even going to address the BM issue because as much as that's a piss off the lying SC is by far, the most pressing issue that you need to get away from right this second. 

that you need to get out. Once a troubled skid crosses that line and starts making false accusations, it's a dealbreaker. Remember - as an outsider, you're the easiest target.

With kids like these and parents like these, you have to protect yourself and your livelihood. You don't have to offer yourself up as yet another sacrifice on the alter of the Failed Family.

We've had other posters share their stories of being falsely accused of all sorts of things by manipulative skids. It CAN happen to you, and even if the skid eventually recants, the loss of reputation and emotional trauma inflicted can't be erased.

Please, please be proactive.

I hope that you find this to be a good place to vent, contribute, and to pick up some useful advice and perspective from others who are living the adventure of the blended family dream.

Like so many others, you are of the mind that "I know his children have to come first".  In my opinion it is rather than kids come first that there is only one top priority and that is the adults at the core of the marriage and the marriage itself. For both of them. Kids are not the priority nor do they ever come first. 

Kids are the top adult responsibility for the adults heading that family/household.  Priority and responsibility. Two very different things.

Because he puts his kids first, by default you nor your relationship are first. When that is the case, it is only a matter of time for the end of the relationship to arrive.  A couple cannot live their best lives together when their relationship and each other are nor first.  

You and he are equity life partners.  That also makes you equity parents to any minor children in your home, life, and marriage regardless of kid biology.

Based on your current thoughts it it being done, call the locksmith to re-key the locks and put he and his failed baggage out of your home and life.  If you choose not to end it I advise strongly that you set and enforce standards of behavior and standards of performance for your SO, and his spawn.  Fist among those is an inviolable stance that his XW can never enter YOUR home for any reason ever. PERIOD! DOT!  IMHO.

You have to commit to living your best life whether you stay or not.  If you cannot do that in this relationship and with a life polluted by his failed family baggage, you owe it to yourself to cut your losses, take a do over, and get on with living well.

Good luck to you and never forget to take care of yourself.

All IMHO of course.

Needs mental help,  Majors mental help by someone with a MD after there. Name.  You have to get it on record that she is a nut,   The BM. Coming into your home is a totally different problem.  That's on DH.  He should of called the police on her . 
'You need to move into a new home where BM has no ties to.  With the understanding she is not welcome there. 
'this is not hard.  Mo st SP don't let there ex into there homes.