What should we do about a relative of ours who refuses to separate from his wife, after it became clear that she is his sister through رضاعة (nursing), and after he was informed of the invalidity of the marriage contract, yet he believes there is no harm in that?

If a man is suckled by his wife's sister five full sucklings within two years [of age], she becomes his maternal aunt through suckling. It is impermissible for him to marry her, and his marriage to her is invalid, and his relationship with her is forbidden. He must separate from her immediately. Furthermore, he should be advised and reminded of Allah, and the guardian or his representative should be informed to separate them.