I agree with Mariner about Meredith and her step mom. The step mom is looking at Meredith as a new part of what has always been a happy family for her. But Meredith is looking at the step mom as a part of the family that her dad chose over her. When you've grown up for the most part without a stable loving traditional-structure family like Meredith did, it's not going to be natural to just jump for joy and get right in there because one wants you. I can totally understand the reactions on both women's parts.
It also totally made sense to me that Christina snapped at Meredith for defending her. It's in her character. Christina recognized that what she did was wrong, and wouldn't feel right just getting a free pass from everyone for it. But Meredith's second defense, listing off all the bad things everyone else had done which Christina forgave was fantastic.
But my absolute favorite scene of the night was at the end, when Christina got on the elevator with Burke and Derek to give her support to the boyfriend she wasn't speaking to.