I'm so excited! Another little Cambridge royal :3
For a brother or sister to George Alexander Louis & Charlotte Elizabeth Diana I think possibly David, Michael, or John, and Alice, Eleanor, or Victoria.
My combo choices: Michael Philip Charles/Michael Henry Charles & Victoria Carole Anne/Victoria Caroline Anne
(The Queen's children are Charles, Anne, Andrew, & Edward, so those are most likely out as first names, as is Henry/Harry as that's William's brother. I don't think they'll use Philip as a first as that's the Queen's husband's name, but it might make a middle name. Here's a link to the wiki page; the family tree is very interesting, and you can click on names there to see the wiki pages of the royal family members.)