My MIL does the same thing but on the other ends of the spectrum. I happen to be open to the idea of an epidural and she keeps telling me that knowing that every contraction will get me closer to the baby should be enough to get me through the pain.
My mother gave me the best advice for dealing with unsolicited labor advice: smile, nod, and lie if you have to. She won't be in the delivery room with you. You and your partner are the only ones who have to know what really happened. It's like politics and religion: everyone has their view and you will never be able to convince anyone that you are right and they are wrong.
As for the guests, they whole smile, nod, and lie thing doesn't work so well. Good luck. Stick to your guns.
On the gender issue, my poor dad is at the other end of the spectrum. As soon as I found out that I was pregnant (and was only 5 weeks), he asked if I knew whether it was a boy or girl. :) I thought it was cute.