Sister in Love with My Husband's Cousin

I have sister who is 24 years old. My sister is in love with my husband's cousin. I knew this when I saw her emails (I found some emails from him). The problem is that he is married with a 9-year old son. My sister is well-educated and settled.

Should I talk to my sister regarding this? I am afraid about how she might react. Should I talk to my husband's cousin?

Please give me suggestions. I want to resolve this. I don't like this because my husband's cousin is 12 years elder to my sister.

I haven't discussed this issue with my husband. Should I talk to him first?