SEOUL, South Korea — The powerful sister of North Korean leader Kim Jong Un vowed Tuesday to boost the country’s nuclear war capability and take other steps to protest the recent arrival of a nuclear-powered U.S. submarine in South Korea. North Korea has repeatedly vowed to expand its nuclear arsenal, but the latest threat by Kim Yo Jong came after North Korea dialed up regional tensions by unveiling a uranium-enrichment facility and testing a new ballistic missile earlier this month. In a