I'm not sure about that. After all, it's really none of our business.
Since they are the ones getting married and are happy about it, what problem should there be?
ご質問ありがとうございます。
① "I'm not sure about that."＝「それに関してはどうでしょう。」
"After all, it's really none of our business."＝「どちらにしろ、私たちには関係ないことです。」
② "Since they are the ones getting married"＝「結婚するのは彼らであって」
"and are happy about it"＝「彼らがそれで幸せなら」
"what problem should there be?"＝「何か問題はあるのでしょうか。」
ご参考に。
アメリカ合衆国 
