My mother in law is planning to throw a party the day after our wedding. Her...

My mother in law is planning to throw a party the day after our wedding. Her initial intention of the day is to invite those who aren't invited to the actual wedding, as she feels they should be invited but we don't wish for them to be a part of our special day. She's decided to hire a marquee, have caterers, have a florist do the flowers, have a live musician for the day and then hire a band for the evening. She's planning on inviting the whole wedding guest list to her party. I feel really upset and quite angry that she's decided to throw what seems to be a second party as an extension of my wedding. I wouldn't mind if it were for close family and friends, and more of a casual party (not hiring caterers/band/florist!), and to expect the whole wedding list to attend (near 100) I feel she's trying to overtake my day. And to invite people that aren't invited is rather embarrassing, for her, I think. My fiancé and I aren't planning on being there as I'd rather rush off on our honeymoon than endure a 2nd day with the whole crowd. I would love for people to enjoy the day, and not feel it's 'gone on' for an extra day too long. I've spoken to her about my feelings, but she's assured me it won't be too much bother and people won't be too tired. I just feel it's too much! Please could I have your opinions on this and any advise you can give to try and stop this rather unnecessary continuation of our wedding. Thanks!