Hi Rabbit B (and Aussie Pom, if you're still reading this thread)
I think I've found Elizabeth and Joseph Burns, the original subjects of AP's request. It looks like they arrived in Melbourne aboard the Morning Light on 26 July 1864 with 18-year-old elder brother (I presume) Hugh Burns. This was a full nine months after their mother and siblings arrived. Elizabeth would have been eight and Joseph 10 at the time of arrival and I dont know how common it would have been for children to remain in the home country with an older sibling after their parents had emigrated.
Elizabeth married a David John Powell on 1 December 1872, with the consent of her mother, given she was not yet 17. She died in February 1944 after having a dozen children.
No idea what might have happened to Hugh or Joseph and if anyone can help, I'd be grateful
GazT