回答
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(Our city) is in the ○○th year of a sister city relationship with ○○ city
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(Our city) celebrates ○○th anniversary of sister city relationship with ○○ city
「○○年目を迎えた」は
"in the ○○th year of ▲▲ relationship with ~"
という表現が使えます
よく見かける "celebrate ○○th anniversary"(「○○周年」)という言い方でも表せると思います。