She didn't HAVE to do that, you know. A lesser person would have let him die rather than give up half of their life force. That's a huge and very long term (possibly permanent) burden to take on.
The fact that she was willing to make that sacrifice for her precious adopted brother is indeed one of her good points, and she definitely did because she loves him as much as she did it because he saved her.
And with that, her debt should have been paid......but she brought him back to the mansion in order to protect him, because she loves him.
I'd go so far as to say that no one in Tsukihime cares more for Shiki than Akiha.
(and Arcueid and Kohaku are my favorites)