So my FI's family is basically paying for our whole wedding (except the ceremony & reception rental paid for by my step-mom). Because of divorces and re-marriages and half siblings I have a VERY large immediate family (brothers, grandmas, aunts/uncles 1st cousins creeping around 40 people) all whom I see on a regular basis and all who would normally come to birthdays and Christmas (Yes we are that family).
My Bio-mom and her family (who I am not close to but they feel close to me) have offered to pay for the rehearsal dinner, in thinking that they could spend some quality time with me and my FI before the craziness of the wedding the next day. I am not close to my mother's family (raised by my Dad, who passed away 3 years ago), and I am planning on only inviting said aunts/uncles & cousins to the reception because we have a wedding venue that holds 100, but a reception site that can hold 300.
How do I let my bio-mom know this before she enlists everyone's help? My mom and her parents will be attending the wedding while her sisters and families will be invited to the reception. This is a very sensitive subject with all the marriages, divorces, current family issues, and losses that have occured on my Dad's side.