What if you and your best friend have been through a lot and you could almost lay down your life for her?
What if your best friend has been complaining that she suspects her man to be cheating on her but she has no idea who the girl is?
What if you and your best friend have planned to find out who the girl is and deal with her?
What if you find out that the girl your best friend's man has been sleeping with is your younger sister?
Will you tell your best friend or will you lie to her? Will you protect your sister? What would you do?

Do u want ur sister to be sleeping with man who is already sleeping with your best friend
Answer above question and u have ur answers

If you dont tell her, then u are the one cheating on ur best friend

If dat is d case, u'll hv 2 confront ur sister cos she's exhibiting early symptoms of oloshoism. Call a family intervention on her if dats what is takes.