Actually OP, i think if you want someone to come all the way from Oz, only giving them 6 months notice isn't really fair when you konw you'll be getting married then now. Why not call your DB and find out when in Summer 2015 (as in, UK summer not his!) he could come over and arrange around that. If it needs to be a cheaper, smaller wedding so you can do it when he can be over, so be it.
for holland, that's not that far or expensive, if people can stay with you/your mum, I can easily see they could do 2 weekends over 2015 to weddings without much drama, it's your DB that'll be the issue as he probably can only come over once.
Get your date set at least with him, talk to him about what suits him/his family (does he have to fit round school holidays there? Could he come over in June with his family even if your mum wanted him too?). And then if your mum wants to arrange her wedding around yours, so be it.
But you can't refuse to arrange yours yet then be annoyed that you'll have to fit roundyour mum's dates if you want others there, you have the opportunity to do the opposite, to arrange first and let your mum fit round you.
Get it done. Few venues will expect a large deposit before 9 months before the wedding, so you might not be any worse off than if you arranged later on, but if you leave it you might have less choice. Summer weekends are busy (and you'll need a saturday wedding if you want your Holland based family to come over easily, as it is they would probably have to fly over on the Friday, back on the sunday, if you minimise the time off work for them then they are more likely to come), leave it any later and you might be limited on venue options anyway and not be able to get cheaper ones.
Jobs for this weekend, call your brother, call some venues! Stop worrying about your mum! (she might want to do an overseas wedding anyway!)