hope someone else will be along soon tilly - my ds is four so an 11 yr old girl somewhat outside my personal experience..however have worked alot with kids, and one thing they can never have too much of is positive attention from parents, so your sis simply cannot go wrong if she does this
The outbursts do sound a little immature for a girl of 11......
so sad though that just last night she's still asking for her parents to be together, it's so in their hearts and souls, isn't it
I think she needs loads of listening to. Your sis did well to ask her last night what would make her happy, she needs to keep the channels of communication open.
And fwiw, I think treat her outbursts as she would a toddler and give no attention while they go on, and let them achieve nothing; if she gives in and changes things in response to an outburst then of course they will continue.
But maybe she is just emotionally a little immature...wouldn't be a crime, or surprising, given that she is from a separated family. Encourage more responsible behaviour by giving her some responsibilities sometimes - not chores she'd hate but something she'd love, if you can think of anything...she is growing up and doesn't need to be treated just as a child now, so she should have some say in her life in some way.
Just brainstorming really, thinking of things that have helped other kids I've known..hope something useful!
btw you are a lovely sister to be so thoughtful about your neice!