IRTT.
OP, you have done absolutely nothing wrong. I feel you have also been unfairly treated on this thread. I’m another one who doesn’t believe a child will need adult therapy as a result of sharing a bedroom with a sibling.
I also have an August 13 year old DD and a 4 year old DD. We live alone after having left a huge house. Due to less tragic circumstances, but difficult ones. So don’t underestimate the impact of that on all of you... whatever it is, it’s clear to understand from what you haven’t wanted to say that you have passed a very difficult period.
In my situ, we have the 3rd bedroom but it never gets used as they either want to be together or in with me... We also have other 13 year olds over to stay... They are all very similar. Moaning about what they don’t have and comparing. Never ever looking at what they do have. In your DD’s case, a safe and loving home with her mum and sister, in a nice area (does matter) as well as travel experiences and activities such as Ballet.
I could write a bloody book about what I listen to from these girls I encounter (I include my own in this) I laugh it off now. I know she is safe, loved and provided for. It’s enough OP.
And to those questioning the second child over space. I can bet OP’s eldest will be far far more thankful for her sister, than having her ‘own’ bedroom, in 20 years time.
I live abroad, I know plenty of wealthy families whose children share rooms in city apartments. All very normal. Then take off travelling in the summer holidays. No care to kids having own rooms etc.
Ps. I’ve not allowed DD to watch pretty little liars after a friend commented on attitude of her DD after watching it. May also be something to consider after the double bed and en suite comments.