Am I Taking This Too Far? My Best Friend Refused To Share Her Own Husband With Me?

I love my best friend like a sister cause we are both the only child of our parents and we grew to love each other so much.

Went to same school till our university level and did our NYSC together . Their was no me without her to the extent that we made love and shared our men, yes we were that close . 

Life was so beautiful with us until she found love and decided to get married, for the first time my best friend refused me sharing her man with her so I respected her opinion. 

They got married and am still dating but got engaged few months back.

I got engaged to one of my ex who we came back together and things worked between us , my wedding is next month but I just found out that my best friend has been cheating on her husband with my boyfriend. 

I caught them red handed in the car ba.nging each other without a c.ondom, I was mad but refused to act stupid, when my best friend came to apologize I understood with her because we have both shared him in the past , but why my husband to be? why she refused to share her own husband. 

So I gave her 2 conditions, either she allows me sleep with her man or I tell him that she cheated .

She has refused so I think I will tell her husband,As for my wedding I will not call it off , I will still marry him but I must cheat in return .

Last night my mum told me that am a wicked friend and am going too far , that is why am sending this story to get public view on this matter .

Am I taking this too far?