I know what you mean.  I love sleeping with my kids and I know it is safe, but there are so many people out there who think it is weird and/or dangerous.  Thank goodness my relatives are okay with it, but I feel I have to hide this from some friends.  They already think I'm strange for some of my other attachment parenting things (like extended breastfeeding).  I'm not sure what you should tell you mother-in-law - maybe give her and your husband some of the literature that shows the safety and benefits of co-sleeping.