My 17 year old dsd appears to be becoming more and more insecure as time goes on.
I've been in her life for over five years. Her mum has had two partners in that time although neither have lived with her officially. I met dp over two years after they split up. She lives with us a third if the time along with her youngest sibling.
18 months ago my dsd2 came to live with us and I think the pecking order has changed dramatically.
She's the eldest of four siblings and one step sibling - my ds.
She has had more opportunities and possessions than her siblings or my ds both from us and from mum's side. That's ok because she's the eldest.
What I am really struggling with is her constant snidey comments. We had a massive sulk last week because she found some smoothie and different brand yogurts in the fridge at ours. Apparently we are denying her "nice food" and dsd2 and ds get it all when she's not here!
We always try and do "family" things when her and her siblings are all together but obviously life does go on when they're not here. I always do a roast, and nice dessert, for example on the weekend they're all here and not the rest of the time. I make her homemade soup because she likes it etc
We decorated her room first when we moved. She's having 4 holidays next year. She has the latest phone. She always seems to wangle a lift from everyone where the others always walk. She has a laptop, a roomful of clothes... Like I say, she really doesn't miss out. She also gets plenty of one to one time with dad which I encourage.
When we were putting photos of the kids up I ended up in a bit of a state worrying that I might get one less picture of her up than the others and I KNEW she would pick up on it.
My ds only has one home, a dad that has promised him 6 times in 2 months to see him but let him down every time. Has to walk, has second hand phone etc etc. Whilst I appreciate that is not dsd fault, I feel slightly annoyed that he's not even "allowed" a yogurt without dsd1 feeling jealous. Dsd2 is also treated like ds although she doesn't get the crap from her mum that ds does from his dad. 
And just to add the situation, dsd1 is CONSTANTLY telling dp and I how great everything is at mums. From mums cooking to her bedroom to mum's boyfriend. She is also very arrogant and often very rude to her siblings and my ds.
A few weeks ago I decided to detach from her as it was getting to me. It has helped massively and made me feel a bit more empowered (I had plenty of times where I have felt intimidated by her). However, I'm still wondering where it will all end? She's struggling with schooling and dropped a lot of subjects she was studying. She changed her mind week to week what she wants to do when she leaves and regularly lies to us to make herself appear like someone who is perfect. She has lost many of her friends. She will hide any vulnerability and only seems to communicate with us to tell us how much better she is than everyone else. I've tried to be a good female role model to her, although I've given up now. I don't feel like I've achieved very much on that front!
I feel like nothing we can do would be good enough anyway. It's very hard to live with.
Can anyone relate to this?