My sister is having an affair

My sister, who is married and a professing Christian, is having an affair with a married man. She hasn’t had any intimacy with her own husband for the past decade due to his longstanding illness. I might have the courage to tell my sister’s husband about the affair if it wasn’t for our parents. They are likely to react very angrily and cut my sister out of the will. My sister keeps excusing what she is doing due to grace! She says I can’t tell her anything that she doesn’t already know or hasn’t thought of. I feel very bad knowing what I know but don’t feel I should spill the beans...