Advise?

It was so funny,when someone left a message in my old blog. I hardly read and update it. Due to the reason that i only want to maintain one blog,instead of having five various blogs. Anyway, here was the comment that was posted in my advise section blog,and i do not find it dodgy. I suppose someone can give her an advise."From Belle: what's up. As i was scanning your blog, some of your entry were somehow related to me. But, i guess this is the only way out that i can do. I am Belle and i have a sister,and we were the child of a merchant. My sister got married when i was 13,and so, as happily newly wed couples,they were,which mean,her husband and my sister need to separately live with us. They have five gorgeously kids and four of them are in secondary year now and one is in college. In the year 1995,their wedlock life seems to be unusual. Her husband started to have a relationship with her fifteen year old friend daugther. Ofcourse no one knew,since he was older than the kid. And so they had a child and that year was a tremendous year to my sister. Seven years passed and everything seems to be good. But i recently foumd out that, the guy,which is my sister's husband,was still having an affair with that girl which is twenty two years old now. And i have a dodgy feeling that they meet sometimes. What should i do,do i need to tell my sister about it or remain innocent. Cheers!