Presupposing that George is gay, then Charlotte, and Charlotte's children will be his heirs, exactly as Harry was William's until William had George.
His being gay is neither here nor there, but, to inherit directly, any children would have to be both his biologically, and legitimately born from his marriage. Currently, the monarch must marry in the Church of England, and the Church doesn't acknowledge gay marriage. Marriage outside the Church wouldn't be recognised, so he would neither be able to marry any future partner to make them any sort of consort, nor have any legitimate heir.
Line of succession is just that - right descends down the line (previously only the male line) from the current Monarch in the most direct route, eldest to eldest. If that line 'runs out', then the line moves back up to the last unbroken place, and then down the next eligible path.
So, Charlotte would (and is until he has legitimate children) be George's heir, not because she's his sister, but because she is the next eligible line from William.
In the incredibly unlikely event that all three Cambridge children don't have a legitimate child, then the line will move back up through William to Charles, and down again to Harry, and then Archie (and Archie's kids, if they count) and then Lillibet and hers.
If both of them don't have legitimate children, either (or do something else constitutionally iffy, like conversion out of the C of E) then the line would go back 'up' to Elizabeth, and then down to Andrew and Beatrice and Eugenie, etc.
There's always a 'line' - and it's publicly available out to about the 200th place. It's quite an interesting read if you're into this sort of thing, and the current one does actually show some good examples of the 'back up and down again' rules, because things did get a bit fragile after Edward abdicated, and left George with two girls as his heirs.
The key is descent from the Monarch, via the shortest legitimate route, and the last update (prior to the 2012 change) was the current Queen's grandad, around WW1.