*Pick up phone
*Dial SIL phone number
*Say hello, but do not enter into small talk
*"SIL, I am not very sad to have to say this, but I do not wish to have you standing up beside me on my wedding day. I don't feel you are a friend to me and so I will not have you in my wedding party. Thank you, but your services are no longer required. Good night."
*Replace reciver
*Pour glass of wine.
I'd only add - *pour glass of wine first and drink.