This takes place around chapter 304.
"Captain Kuchiki, I've finished treating you. But please don't stand up for few minutes," said Isane Kotetsu to Byakuya Kuchiki. "You as well, Rukia-san, please rest for a little while longer," she said, turning to a small woman who sat quietly, hugging her knees. "Hanatarou and I need to report back to Captain Unohana. Is it all right if we leave you here for a short while?"
"Yes…thank you," said Rukia.
"Captain…?" she said, turning to Byakuya, who nodded. "We'll be going then."
Rukia looked down at the desert floor of Hueco Mundo, tracing a little circle in the sand. She closed her eyes tightly, trying to shut out all the guilt that now flooded her. If her abilities were stronger, if she hadn't let herself get to the point where she needed rescuing, her brother wouldn't be sitting there injured.
"Nii-sama…I'm sorry," she said.
"Why is that?" Byakuya responded.
Rukia sat up, confused.
"Well…if it weren't for my inabilities as a shinigami, you wouldn't be-"
"Rukia," Byakuya cut in, "stop saying such things. I promised…that I would protect you. How I go about that should be none of your concern."
"Please let me finish," Byakuya said.
Surprised, Rukia turned to look at her brother, who stared into the distance as he spoke.
"My responsibilities as a captain have undoubtedly increased since Aizen defected from the Gotei 13," he said slowly. "I hardly have time to think about anything other than the tasks at hand. However…" he paused, "in certain instances, if only for a moment…I instinctively wonder if you're well, if you are happy…how Kurosaki is treating you in the real world. It is really quite troublesome…" he added thoughtfully. "This is not because of the promise I made. This is because you are my family…and although it is merely troublesome to worry about whether you are happy…to worry about whether you are alive…is simply…" he trailed off.
Rukia stared down once again.
"Thank you…nii-sama…for saving me."
"Again," Byakuya said, "do not say such things. If you should be at all thankful…be thankful that you will live to save your friend…and I will be thankful as well…that you will continue to trouble me."