Final answer:
The phrase 'made it up' in the statement is an example of an English idiom. It indicates making amends or reconciling after a disagreement, in this case with the speaker's sister.
Step-by-step explanation:
In the statement, "I made it up with my sister", the underlined part 'made it up' stands for an English idiom. English idioms are phrases or expressions used in common parlance that convey a figurative meaning different from the literal interpretation of the words used. Here 'made it up' means to reconcile, mend or fix a relationship with someone after a disagreement or fight. So, in this context, it means the speaker resolved some issues or improved their relationship with their sister.
Learn more about English Idioms