Yair Mor, whose brother, Eitan, was abducted from the music festival in Re'im on October 7th, enlisted on Thursday in the Border Police.
His family accompanied him to the conscription site at Ammunition Hill in Jerusalem where he joined the service.
His father, Zvika Mor, a member of the hostages' families Tikva Forum, stated: "We are all obligated in this mandatory war for the defense of the nation and land, no one is exempt."
He added: "Besides this total obligation, it is also a great privilege for us, one that our ancestors could only have dreamt of. We wish Yair and his entire draft cohort - be the best you can be. The people of Israel trust you completely."