Siblings?? Gillian said that once about them, and right away laughed at herself for saying so. They’re everything but siblings, and you really can’t take away their sexual chemistry. It’s there, either you want it or not. Actually, either THEY want it or not. It’s just there.
Gillian can’t control her heart eyes and sexual innuendos when she’s around him, and he can’t help being territorial, touchy and over-protective when he’s with her. I know it’s been a long time since we’ve seen them together, but I suggest you rewatch some one their previous joint appearances and wait for the next ones. If you still see two siblings after that, there’s nothing I can do for you!