Translation of "Mistress" in Japanese

Between ourselves, he keeps a mistress.

ここだけの話だが、彼には愛人がいる。

There was a provision in his will for his mistress.

彼の遺言書には愛人を相続人とした条項が存在する。

Also decided to bring along his mistress, poorly disguised as an officer of dragoons.

がドラグーンの将校になりすましていない彼の愛人も連れて行くことにしたことに

I'm afraid of discord arising with my husband so I'm turning a blind eye to his mistress.

夫との間に波風が立つのが恐くて、愛人の存在は知らない振りをしています。