Only a little bit of advice I'm afraid, but plenty of hugs and understanding

At the ripe old age of 21 I somehow wound up step-mother to a 14 year old stepson and 8 year old stepdaughter while pregnant with my DD. Queue their mother having a hissy-fitting breakdown, deciding she couldn't deal with the kids anymore and they could come live with XP and I for a while. Stepson was fine, actually quite easy to get along with but stepdaughter was a temperamental, manipulative little ****.

The only thing I could do was to stick to my guns about acceptable behaviour and make sure that XP backed me up 100 per cent. Sit down with your DH and discuss what is absolutely not-negotiable (lying) what might have a little wiggle room depending on circumstances and what is up for negotiation. Then sit down with your stepdaughter together or with the family as a group to discuss this, making sure everyone is aware of the expectations and also the consequences for non-compliance. That way no one can say they didn't know or wasn't told.

All the best hon, hope it works out for you all.