Hi,
not in quite the same situation but I do have a nearly 18yr old and 21yr old who are not over the moon about the move (we are so mean making them choose between thier family and thier friends) but they are going to come on our visa and hopefully stay until we gain citizenship, then they will have choices
we have been talking to them about the fact that at different stages we change/lose/gain friends, the 18yr old will be leaving college and going to UNI and the 21yrs old will be completing 3rd yr of UNI (hoping things are in place so that one will start degree and the other will complete masters in OZ) both times when friendships change however much we think we will stay in touch, we drift away from people we are no longer in regular contact with.....
Also hubby keeps saying how will they support themselves if they stay behind, there will only be my brother left in UK and nobody likes my SIL
It's a very hard thing to have to think about but I think the suggestion that they come for the short term with the option to go back when they are older and have citizenship is a good one
Are you and ex able to talk about what is best for the girls? do you think he would be on your side?
Would you able to send them back to UK for regular visits?
My niece who is now 19 went over in the Nov following her GCSE's so she had just started at college and was not happy about the move at all,

tried all sorts to stay in UK but went in the end. She had to start back at school as they do not leave school until 18 which again did not go down well,
BUT I asked her the other day if she thought Oz was a good move and she said it was, she loves her lifestyle over there, lots of parties, going out to dinner with her girl friends, even working part time, she does not think she would be getting such a good rate of pay in UK and she has just started Uni which she also thinks is great
She has made lots of new friends and she is still in touch with some of her friends from UK

sorry a ramble, hope it is of some help