For struggling to accept this horrible situation with SK

Stop beating yourself up over his ex making malicious reports, keep your children safe and don't go near them. I bet they are grilled after the visits as to who they have been with or seen, so make sure to stay clear.
My worry would be that eventually the ex will lose the plot and the kids will end up with their father full time, it would turn your home upside down and I don't think will be a happy little family as they have been coached by the mother to be nasty.
You may want to think about what you and your children will do if that happened.

Agree, and would stop with the buying presents, their mum could easily twist that bribery and coercion.

I would be glad they are nowhere near my child, and would insist on the set up you have, and demand it stays that way and keep them away from both my children !
Putting you own children first is to keep them away from them and stop worrying about lovely blended family.
This works better for you and in time it will be them who are unhappy with the set up.
They'll prob want to come less now as they get older anyway.

Absolutely, what a mess, why do people keep having kids all the time. It's so sad for them.

They may be the unwitting instruments of a troubled and trouble making BM, but you will be doing them a favour by instilling in them now the knowledge that making false accusatIons is not the way to behave.

I think this is very wise and whilst I would definitely be keeping them separate to ds it might be helpful to reframe it to bear in mind that they are children too, who are probably being manipulated and in an emotionally charged and difficult situation that they haven’t got the maturity or skills to navigate.

Shouldn't it just be Mum rather than biomum?

The birth mother?

These aren't adopted children, they're children who live with their mother.

The OP isn't their step mother; the girls are her partner's children who don't live with her.

The girls are also almost certainly being abused or exposed to age inappropriate sexual content somewhere, whether at their own (mother's) house, at the OP's house or somewhere else. Does their mother have a boyfriend/partner/husband? Are there other older boys living in their main home amongst the older siblings (and maybe there are more "blended" pseudo-step siblings /relatives/ children of mother's partner) referred to?

Stop contact with those two girls immediately. Your DP must see them on his own. You don't need that in your life.

You don’t have to accept these kids. They really are no relation to you and your priority should be your kids. There’s no way I would have anything to do with them, especially not buying them gifts!!
I would even go as far as walking away from this relationship as it just sounds like a lifetime of misery and issues down the line. After some time passes your dp will start downplaying what they actually did, and you both will start resenting each other as he ‘forgets’ and your poor ds will bear the brunt again.

Its more likely that the next 5 or 6 year could prove the most difficult. When they start falling out with the birth mother and suddenly find themselves homeless at 16 or 17.

OP both SDs have 2 parents looking out for them.

Your DS only has you. He must be your priority even over your relationship. Keep them well at arms length from you and your kids. Stop buying them presents as that gives a poor message to your son. Your DP can buy them presents from him so they won't miss out.

Its very sad they won't have a relationship with their HB but given the 12 year age gap thats neither here nor there.

This I am afraid. Your 'happy blended family situation' has turned out to be anything but; it suited you as one of the adults involved to believe it was happy, that's all. The children aren't happy though.

Although nothing can justify their making false accusations against an innocent small child, I understand that you feel for the children of your DP as well. I hope they receive support, kindness and understanding.....but it can't come from you in this situation. They have parents of their own and SS's involved, so leave them alone now. And don't send them gifts, that could be misconstrued and might even put your own DS at further risk.

You need a laser focus on your DS's interests now; just protect him from all this drama that has nothing to do with him.

A 7 year old that's sexually abused 2 teenage girls? I'd imagine if such a thing was to happen then it would be a child who was abused themselves and doing it to those his own age or smaller

I honestly think that your current arrangements are best for all the children. You are removing your stepchildren from a situation in which they can be manipulated and pressured by their mother into telling lies that could haunt them as adults.
You are providing a consistent arrangement for the girls and few things could be more valuable to them now. And this arrangement is safe for your other children too.

I understand why it doesn’t feel good because it’s not what you want for your family. But it fulfills your most fundamental obligation as a parent which is to keep your children safe. And the situation you wish for is simply not one of your choices so going back to the basic parenting principle of protection is the best way forward for you all.

You sound immensely compassionate and level-headed in the face of very difficult circumstances and I think you should trust your instincts. Whether she is motivated by mental illness or just a poisonous personality, your DH’s ex wants to create drama and division and calmly refusing to let her is actually your best revenge. People like her hate to be dismissed.

but you have a happy family unit @MangaMoo; your 2 sons and your partner.

If the current arrangement wrt to your partner and his daughters is working, then leave well alone.

Just concentrate on your own family and do not get involved with the girls (gifts for them will change nothing)

Protect your boys and please, please ensure that you are financially independent.

You weren't a 'happy blended family' though - that was in your head. In reality you were a random grouping of people who didn't want or choose to spend time together. And that's evident from the very dangerous and horrible situation that transpired.

I'd personally be taking my son out of this fucked up situation, leaving my DH and having shared custody with the baby only on the proviso the baby was never left alone with the daughters.

Just to say, as an ex social worker, I have been aware of 7 year olds who sexually abuse. Very unusual, but it is possible.
I'm not saying OP's son is a sexual abuser, just that it can occur.

Do SDs know that the allegations were found to be false and malicious? I appreciate that they are only children themselves and obviously have loyalty to their mother, but they are old enough to have the seriousness of false allegations explained to them and to understand.

Aren't they keen to see their baby brother?

I'm not suggesting anything will change immediately, but it could be that in a couple of years they will realise the awfulness of what they caused, and tell the truth.

In the meantime, of course I understand why their father still sees them but I wouldn't be sending them presents, and if they asked why they can’t come to your house like before I hope he is telling them exactly why.

they are regularly used acronyms since the advent of parenting fora on the internet.

And even if you don't know - context helps

No advice but just wanted to say that you sound like a lovely caring person who is seeking to understand why you SDs are behaving in this way which is very much to your credit given how horrible this must be for you and your son.

Does your DS have a relationship with his father? And any custody arrangement?

Couldn’t follow because of the acronyms.

I'm sorry to say this but the relationship is clearly not working for your child so why are you still there? It sounds an extremely toxic situation and your son needs out of it.

Something has happened to your partner's daughters if they thought of accusing a seven year old of sexual assault OP. This kind of sexualised behaviour is a red flag for sexual abuse or exposure to inappropriate sexual content coming from somewhere in their lives.

The entire thing sounds like a mess but the 12 and 13 year old DDs of your DP (they're not really your step daughters) are quite possibly at risk from someone in their messy and chaotic lives and are the ones their father should be concerned about, if he's a good person.

At 12 & 13 they knew what they were doing.
Keep well away from them, & certainly don't send presents - that could be misconstrued.