You can't unlive the harm that the false allegations did to you.  I wouldn't want the girls in my house.  You could risk losing your own child.

Seems like Mom may be reaching out for help because she needs help to clean up the mess she helped to make.

Tell your DH to take baby steps. Sincere apologies from the girls for their behavior towards your house, see them outside your house, family therapy.  Only after they do the work should you even consider having them in your home.

If I were you, I'd step away and let your DH handle all of this. You are too kind for your own good. Block BM's number - yes, she was apaologetic but she's using all the means at her disposal to reel you back in so that you take some of the weight off of her. REMINDER: these are not your kids and not your responsibility. 

Do not allow the SDs back into your home ever again. Let DH see them outside of your home but do not let them back. A CPS visit is no laughing matter and it's not as if these girls were too youing to know what they were doing. They knew perfectly well what they were doing. He can see them and spend time with them but they have burned all the bridges that led to your home. 

Stand your ground. They have caused you grief, cost you a ton of money in legal expenses, and could have potentially lost you your daughter. No is a complete sentence. Besides, if I have understood correctly, this is all about BM, right? Her daughters haven't shown any kind of remorse nor interest to see you again ... Don't do it.

Alot of these bio mums after divorce create monsters against their dads and when everything goes to shits, they palm it off to dad and his new wife or dump kid at their front door and drive off.

my husbands exwife is a hcgubm, narcissist and abusive hell bent on pas. After all this and sd's questioning her on the lies she made about me and their dad, has eldest sd23 text daddy a message from bio mum stating that they are now both remarried with new families and basically need to put everything aside for the sake of the kids. 
except she was hell bent destroying any relationship and now wanted to play besties

hubby refused to respond to that message and ignored it. He had no intentions ever going to bio mums home of her affair husband, the guy who was having an affair with bio mum pre separation of hubby and biomum.

bio mum wanted my husband to come there with ss to meet sd's in her new husbands home that her 2 ss live in and hubby was expected to leave me and our 2 kids aged 1 & 2 at home. Hubby wanted none of that. Why did he need to meet his daughters in the affair husbands home she shares with 2 ss's?

hubby told sd he was hapoy to meet at a cafe/restaurant nearby and eventually she did after telling biomum. If biomum was a newly changed woman who sd admitted lied before about so many things, she couldn't ban the meet or forbid it. 
 

It doesn't pass the pub test, something is fishy about her story and her going through a divorce, plenty of bio mums use this to mask their craziness and how they've done so much bad to destroy relationships.
 

My husbands exwife said this back in mid 2018 so she could have a poor pity me party and claimed she was dying of an imaginary illness no dr detected as she was in perfect health. It is now over 3.5 yrs on, she is still not divorced and still has no illness, its all a con

I totally agree with everyone that, whatever your DH decides, he sees his kids outside your marital home. As for BM, I'm no fortune  teller but something tells me her kids are now out of control to the point she can't handle them and wants you and DH to step in to that snake pit. Do NOT do it. Block her. DH can begin a slow process of renewing relationships with his kids but you and your child don't need any part in that mess, as it currently stands. In fact, I'd need to see years of changed behavior before letting those kids around my child, myself or in my home again. You don't play with CPS...you could end up having your own child removed from the home based on their crazy lies. And then face a nightmare going through the court system. Nope. 

Your DH may want to include an experienced counselor (in the high conflict step world) to help manage the reunification process with his kids. Something tells me he's going to need it with the behaviors you've described.

And tell BM.  
 

"The safety of My/our young child will not be jeopardized by exposure to the  dangerous failed first family children.  As a responsible parent to a toddler I will not allow that risk."

Lather, rinse, repeat.

It will never be the same

Your H and BM are the ONLY ones who need to address their daughters' behavioral issues. Your H can exercise visitation outside your home, go to therapy with them etc without involving you. It sounds as if he wants you to reengage to act as a buffer/meat shield, but this is not a good idea. The core relationship exists between father and daughter(s), so that's where the work should be. 

Your situation is challenging because neither bioparent demonstrates strong parenting skills or good judgement. This means you have to stand strong and be ruthless about protecting your peace. There was a real lack of boundaries and grasp of respective roles all around, followed by BM making war and weaponizing her daughters against you. Now both bioparents want to drag you back into dysfunction, but you've changed; you're a mother now, so your priorities have shifted; you're tired of the crazy and don't want to be victimized anymore; and you've laid down some healthy boundaries after being targeted and abused, which is good. Now you just need to stay in your own lane and not allow your H or BM to draw you back in.

Your role was/is to support your H's parenting, not to do it for him. As for the skids, well, some things are just unforgivable. Calling CPS on you is one in my book. You have a child to protect now, so keep your boundaries up.