SEOUL—North Korea opposes war but would use nuclear weapons if South Korea attacked, Kim Yo Jong, the powerful sister of leader Kim Jong Un, said on Tuesday, in a warning that analysts said is probably aimed at the South’s incoming conservative president. Kim Yo Jong, a senior official in the North Korean regime, said it was a “very big mistake” for South Korea’s minister of defense to make recent remarks discussing attacks on the North, state news agency KCNA reported. South Korean Defense Minister Suh Wook had said on Friday that his country’s military has a variety of missiles with significantly improved range, accuracy, and power, with “the ability to accurately and quickly hit any target in North Korea.” Both Koreas have increased displays of military strength after North Korea test-fired a range of increasingly powerful missiles this year. Officials in Seoul and Washington also fear it may be preparing …