Lots of nodding and "Mmmhmmm" while you tune out whatever you aren't interested in hearing.
You are never going to convince mothers and inlaws (who have had actual experience birthing and parenting) that you know best just because you read something in a book. So don't bother trying. Just do things the way you feel is right.
I had a natural birth, breastfed, coslept and all that jazz, while living with my MIL, who had a C-section, formula fed, cried it out etc... I was confident in my choices and let her know that she could support me or stay out of my way. Dare I say I've actually swayed her opinion on a number of things and I've *gasp* actually heard her encourage DH's cousins to breastfeed and delay solids.
I think every new Mom-to-be goes through this in some form or another.