I posted a thread in step parenting yesterday (sorry I don't know which button to press in my phone to ask Mumsnet HQ to move thread), here's a link:
www.mumsnet.com/Talk/stepparenting/2434235-Cant-get-passed-feeling-angry-for-him
Anyway I've been thinking overnight that the problem is really with my ILs and this latest situation is just a symptom of that. My BIL has always been a bit of a snake in the grass when it comes to DH. He will initially make the right noises and seem supportive then set about making snide jibes and nasty comments. He tries to order DH around via text message, which is hilarious as he is the younger sibling. They are phrased like 'you are to do this or you are to do that'. DH ignores it but it does wind him up. If DH says anything BIL will act all hurt. My SIL is very bitchy about everyone and has said some horrible things over the years to lots of people. Their now adult child grew up being pushed from pillar to post because of BILs job, regularly changed schools no stability at all. The child (won't put gender in) recently got into serious trouble by fighting in a pub and narrowly avoided further action by Police but others who were also involved ended up in prison. 
I genuinely don't think it's the kid's (young adult's) fault as over the years I've seen them struggle with fitting in, gaining weight, keeping friends and emotionally stability. When we saw them years ago BIL and SIL had a massive fight in public in front of them, I just put my my arms around them and comforted them as they sobbed. Very sad 
Anyway in contrast DHs kids have done brilliantly! Great grades, top marks. DH spent many hours going through coursework with them and helping them as they are following him in to his profession so he's perfectly placed to mentor them. They have come through with flying colours!
Anyways. ILs took no interest in kids previously, SIL used to make nasty comments about them which was horrible. But now they've started taking an interest in DHs kids and freezing out DH. They are going over for a holiday and almost trying to take credit for them!
I know I should just say great, have a great time but I can't help but worry. I also know a lot more than the ex does about what's going in and I'm worried they'll be stuck in a situation where ILs will be fighting, encouraging them to get drunk or saying nasty things about DH to stir up trouble.
Anyway I'm sure I'm being unreasonable, as I've been told on the step parenting board, but I can't help but feel pissed off for DH and worried for them. They are not the most worldly kids and the ILs can be quite devious.
I've posted here now because I think there might be a lot more experience around IL issues 