Thankyou. That's very interesting and certainly food for thought.
So anyhow as promised an update.
DH went to XW and asked if she, him and DSS1 could all sit together and talk so they took him to a local pub for a drink. They told him to tell his side and they would support him together, but that they both consider there's 2 sides to every story and wouldn't be making any decisions/ taking action until they had both.
Turns out the DD has been goading DSS1 saying he had to move out because she moved in because he is a PITA and they didn't want him there, and that DSS2 only stays half the week at that house because he's a PITA too but nicer than DSS1. And that she deserved his room, because she was better and her dad earnt more money than his mum and paid for everything - which isn't actually true.
She has the smaller of the 2 rooms as DSS 1&2 had to share when DSS1 lived there but apparently she has asked her dad if she could have the larger room now because she's there the most and older than DSS2 who lives there and her dad said he'd discuss it with the boys mum.
So seems DSS1 was telling the truth about this girl re her behaviour. But I guess there may be a root cause because this girl lives with her dad and only gets to see her mum every other Saturday, whereas the boys have 2 families they flit between - and both love it usually. Also we have been a blended family for nearly 5 years whereas her ( and her dad) have just come into it iyswim?
So XW talked to her DP and his DD tonight after DSS1 came home. The girl admitted saying it but still thinks she's right re room and said she'll not stop saying it because she believes she's right. The rest of stuff talked about I don't know what was decided as that's between XW, her DP and the 2 DC's and I've told DH to butt out and leave it that way.
I spoke to DSS1 and told him that I will always listen if something is upsetting him, that I love him and love having him living with us, and that him and his brother are always welcome. But that he has a mum and needs to talk to her about issues in her home, and that if he feels unlistened to he can talk to us but we can't dictate what she does or how she deals with it. I did say he may just have to suck some of it up.
XW rang whilst DH was putting twins down and I answered, so spoke to her a bit and told her what I'd said to DSS1 - she thanked me, admitted she finds the DD hard work. I said that I didn't expect her to tell me stuff that wasn't mine (or DH's!) business but she said her DP agreed his DD was wrong, that he knows she's being a madam and she admitted to her dad it's jealousy because the boys have 2 'mums' and she hardly has one who cares. 
I am due another child in 15 weeks and we have said that before then we will look again at contact, and are looking at DSS1 having mid week contact at his mums on the weeks he doesn't do weekends. We have not mentioned this to DSS1 but feel he needs more time at his mums (and she misses him like crazy
) for him to feel he can communicate with her and see her as equal parent to DH and I. They have a 50/50 access agreement anyhow.
Think this is just a storm we'll have to weather as the blended family re blends!