I would give her some slack
I would agree except that it is the dog which is going to suffer.
DD2 was told she had to take responsibility. She promised she would. Parents and DD! were aware a dog is a family member and it seems that they "did their bit" - ie it wasn't totally left up to DD.
Then circumstances changed. Parents and DD@ now have work commitments which mean they can't do so much and dog is left alone all day. (Which is frankly awful - but to be fair on OP she can't possibly have realised how these things would turn out, or how much the lonely little dog would suffer).
The only one now in a position to walk and provide company for the dog, is DD2 - now older anyway and able to take more responsibility for HER pet. And she won't do it.
It isn't that she has a full rich life socialising, rock climbing, surfing - or even spends time in the library researching Byzantine architecture - she sits in her bedroom gaming etc on her phone and the dog is left - poor thing must be frantic for exercise, bored out of its skull and desperate for company.
It would do this child's mental and physical health an enormous amount of good to get off her lazy bum and take the dog out for a walk. And the dog would benefit hugely, too - as well as bonding with her.
I have no suggestions except - re-home the dog, but to the dog's benefit, not your own OP.
I have three spaniels. I'd consider having yours too if you were near me and she fitted in with the others. But as I say, a breed rescue would have no problems re-homing the dog for you - also - is it neutered? If not be VERY careful where you let it go to, particularly if it is a bitch.