@Toughroadahead what an awful situation for you!
Firstly, congrats on your pregnancy.
Secondly, my overall advice is to step back and detach from all the drama. Your number priority right now is your health and well-being. IVF is a gruelling process to go through and you will still be super emotional. You still have the pregnancy ahead of you. This period should be as calm and stress free as possible (it should be a deeply happy time). Shield yourself from things that take away from that.
Re the behaviour of the SC?
I’m appalled at the behaviour of the 20 year old. An adult in their own right, what makes them think they have the right to
A) Invade someone’s privacy so blatantly
B) I assume this 20 YO has then chosen to tell the 13 year olds and create this drama?
If this was my child, I would tear them a new one. I’d be appalled by the complete lack of boundaries and disrespect.
I also agree with @Tattler2 re the counselling. But for you. Your sentences of it will break me if they turned against me or OH. And the comment of its making you detach from your pregnancy. Detach from them / the drama, not the pregnancy.
Also, I would hope you aim to get to a better place whereby their actions don’t break you. Honestly? Don’t give them this much power.
If your DH/ his children start therapy separately at first, that may not be a bad thing. He needs to shield you from them and their negativity. He’s their parent and he made this commitment with you, it was for him to think through/ handle the ramifications to his children.
It’s your job to look after yourself. Congrats once again.