Eh, I get that biologically she is Rory's half sister, but they aren't at all close for me to think of her like that. Actually, there was that episode where Rory is on a date and the guy asks her if she has any siblings and she says no.
Plus, I hate that as soon as Gigi was born, Chris prioritised her over Rory. Once she is born, there are no phone calls to Rory, we don't see him again until Sherry leaves and he can't handle one day of looking after Gigi, so he has to call Lorelai for help. I know that isn't Gigi's fault, but again, it's what she represents.