Harassment of 13 year old? What would you do?

So, my new husband who I have been with for 5 years (married for 2 months).
His ex wife banned him from seeing his children (without reason) and mediation has begun. Unfortunately his 2 children missed our wedding as his ex moved without warning 200 miles away and it took 6 months to trace her.
He was blocked on all media and she changed her phone number and that of their 15 year old daughter. It was devestating the children weren’t at our wedding (8&15).

So his is where it gets messy. Angry letter received from her solicitor that the children are gutted that they couldn’t be at our wedding and no longer feel part of their fathers life.
We couldn’t understand how they knew we were married (the solicitors letter included our wedding date and location).

My 13 year old daughter with mental health problems drops into conversation, ‘NAME (my stepdaughter), contacted me on instagram last month asking all about the wedding and asking if it’s happened, I told her all about it and how pretty it was, she wanted to see pictures and I sent her a picture and told her the date it was.’
I asked how she contacted her (I blocked her on instagram because my step daughter is autistic and doesn’t use the accounts, it’s her mother). My stepdaughter apparently made a brand new account.
I read the messages and I know how my step daughter writes and it is not her, but definitely an adult, even calling her ‘good girl.’
I’m livid that she is pulling my daughter into this to the point where I am seriously considering wading in with a solicitor because I am so cross that my daughter is naively being used as a weapon by this crazy woman.
What would you do?!

Your post is confusing but get the impression that your step daughter contacted your daughter via social media?

Obvious answer is to not allow, or at least control, your 13 year old daughters access to social media.