Though normally Lot is called Avraham’s nephew, when Avram was told that Lot was captured, it says, “And Avram heard that his brother was captured, and he armed his household...”
The Malbim points out that we see here how important family is. Though Avraham had at most a few hundred people, the love of his family gave him the courage to take on massive armies in order to save Lot.
As Jews, who share one Father, we are all brothers, and we should be ready to put ourselves on the line for each other. We should be strengthened by our principles and not fear, for the G-d of Avraham comes to our aid.
In a similar vein, the Tzror HaMor comments that Avram realized Lot was captured out of malice and ill-will towards Avram and his beliefs. In fact, they thought Lot was Avram’s brother, not his nephew. Therefore, when he went to save Lot, Avram would only take those who had grown up in his home and been well-trained in Mitzvos. Why? Because when it is Avodas Hashem which is being attacked, only those who truly appreciate it can defend it properly. Those who merely follow through for ulterior motives or as lip service cannot properly defend Torah.
