How can you deal with the meddling and competition from your in-laws during your pregnancy? Here are my suggestions, culled from research studies, anecdotal records, and my personal experience as two-time mother. (See, I listened to my in-laws' parting comment and didn't stop at one.)
Recognize the problem. Maybe your sister-in-law has been competing with you from Day 1. Do you really expect her to stop now? Your pregnancy has nothing to do with her attitude; she'll compete over any issue.
Weigh the advice your in-laws give you. Some of it will be remarkably dopey, but among the dross you'll find nuggets of gold. For example, there is a lot to be said for getting rest during pregnancy and after delivery; I wish I had taken that advice! When my daughter was born, I was teaching Tuesdays and Thursdays at a major university. I taught on a Thursday, gave birth to my daughter on Friday, and was back in the class on Tuesday. What a hero! No, what an idiot. It took me a whole lot longer to heal and recover my strength than it would have had I rested.
Chill out. Recognize that your in-laws often mean well. Don't take their meddling personally, even if it's meant that way.
Involve your in-laws in the process. Having a baby is a blessed and joyous experience, excruciating pain notwithstanding. Besides, the sooner you involve them with the pregnancy, the more willing they'll be to baby-sit when you're so sick of diapers, bottles, and baby gas that you welcome the chance to watch linoleum curl or even -- food shop.