It's unlikely that good relationship is going to last very long.
cocoさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、下記の言い方ではいかがでしょうか。
It's unlikely (that) that good relationship is going to last very long.
--- unlikely = 可能性が低い】
--- last = 続く
ご参考にしていただければ幸いです。
The chances of having a continuously good relationship with your spouse or other people are not that high.
この場合、次のような言い方ができますよ。
ーThe chances of having a continuously good relationship with your spouse or other people are not that high.
「配偶者や他人と良い関係が続く確率はそんなに高くない」
the chance of ... で「…の確率」
spouse で「配偶者」
ご参考まで！
ドイツ 
カナダ 
