11F wants to go by her middle name, other children (9F and 14F) aren't respecting her decision and are actively refusing to use her new first name. Wat do?

Basically what the title says. Massive wall of text incoming, just need to vent a bit, I think.My middle daughter hates her name. Absolutely loathes it. She hates that her estranged father (who she hasn't seen for 10 years) chose it, she hates that it's the name of a gaming character and she hates that it is the bastardisation of a name of a weapon.As shes gotten older she has become more and more sick of people commenting about how strange/unusual her name is. We've talked about finding a nickname she likes that plays off her first name, but she doesn't like any of them.For the last two years she's been coming to me with 'new' name suggestions and I've always had to shoot her down because legally I can't change her name without her father's permission. He is the type to get involved and to throw his weight around just to cause problems, so myself and my solicitor have decided it's not worth poking that bear until my daughters are old enough to decide who they want to live with without his input (we're in Australia, btw).So to the point; we've been talking family history lately and my grandfather's brother came up. He is Uncle John, where his real name was Sam and went by his middle name. Collective light bulbs went off, and 11F decided to go from her first name to her middle name. So she has become Jane (fake name).We've talked about all the issues she will face; people at school, still having to use her legal name on documents, etc, but I didn't consider the fact that her biggest opponents would be her sisters.Oldest is opposing just because she doesn't like it when others are the centre of attention (she's a whole 'nother post on her own) whereas youngest just threw the biggest temper tantrum, said it was stupid and she's going to call her by her real first name regardless.This REALLY upset Jane. This poor kid is always being handed the short stick. Her siblings take after the other side of the family, tall, skinny, olive skin, great hair, really pretty features. Jane is dumpy, is already growing boobs, has a monobrow that thick that we've been waxing it since she was 8, and has my gross crinkly hair and cowlicks. Oldest doesn't hold back in pointing out Jane's flaws, and verbally attacks her because Jane's boobs are bigger than hers 'because she's just fat'. Youngest is acting out because she just doesn't understand, and is following oldest's lead.I've told them in no uncertain terms that this is how it is now, they don't get an opinion as it's solely Jane's decision to make, that slip ups are okay but they are to actively try to remember to call Jane Jane. They've ignored me, and I have no idea how to punish them. Devices and video games have been taken off them, TV is banned and the next step is grounding them to their rooms until they get over themselves.I support Jane 110% in her self discovery and her identity search, same as I would with the others if they were going through this. I just am out of my element with this and have no clue where to go from here. Wat do?EDIT: Quick edit to say that counselling is already on the cards, I'm ringing in the morning to get a family appointment with someone at oldest's counsellers practice. via /r/Parenting http://ift.tt/2nsrRvO