What did I apologize for regarding my comments about your sister?

The choice of "taking back my words" accurately reflects the context of an apology regarding comments that were previously made about someone's sister. When someone apologizes for something they said, it often implies that they wish to retract or negate the impact of those comments, indicating regret over having expressed them in the first place. This phrase encapsulates the idea of wanting to undo the previous statements that may have caused offense or hurt.

In contrast, the other choices do not quite capture the nuance of the situation. "Repeating my words" would imply a focus on the act of restating rather than regret, while "what I should have said" suggests a desire to correct or improve the original comments without explicitly taking responsibility for the previous statement. "My misunderstanding" points to a lack of clarity or a misjudgment but does not inherently convey an apology for the comments themselves. Thus, "taking back my words" aligns closely with the intention behind the apologies typically expressed in such contexts.