Your birth pain management is your business, she can deal with her pain as she sees fit.
It sounds like you'd benefit from finding ways to increase your self confidence, so you'll be less intimidated by her criticisms.
My sister in law has been spiteful about me in the past too, accusing me of being bossy to my husband when the truth is she is narked because she can't control us as a couple. I try not to let it bother me because i know what she says isn't true - its just there's a part of me that ways to let everyone else know its not true in case they believe her! I had a lovely surprise this summer though, when a distant family member said they expected me to be nasty (because of reports from sister in law), but having got to know me, found I was nothing like that at all. It put my mind at rest that people get to sniff out the bitchy comments and usually make up their own minds in the end.
I wonder who you're worried your sister in law will bitch to? Other member of his family, or mutual friends? If its family I'd say they will learn who you are over time, and hopefully they'll have a more "live and let live" approach in life.
You say you want to stick up for yourself. I would avoid a slinging match, as no one will stand to gain anything from that, but maybe research assertiveness so you can name what she's doing to her: "owch! Was that intended to be hurtful or do you really mean something else?!!! 
It's like someone kicking you under the table, most people wouldn't say anything, but for those who verbalise what's just happening, the power of stealth manipulative spite is taken away from them.
My guess is she would deny it, and then of course you could say "oh good! I did think it would be odd for my sister in law to be out to get me, so I'm glad you're not like that!"
Or whatever your own words are.
But don't meet her on her level, she would live to get you to be spiteful back so she can report that to everyone else!!!!!
And don't forget those who know her probably will have experienced her being like this with other people in life. I'd be surprised if you're the first!
Finally, have you talked this through with hubby? Having said that, brothers (men) can be blind, especially when women are being subtle in their spite, because they don't operate they way so often can't see its there. my hubby had thankfully become aware over time that his sister is sometimes spiteful to me and he is supportive of me. But it took him a while to stop denying it (she had to step too far to make it obvious).
Good luck though, and try not to be too anxious. Bullies love the idea they can create a fear response in someone. Personally I'd consider refusing to have her to stay if she carries on. Or I'd even vote with my feet and consider going away with the girls if she was coming to stay against my wishes.
You are not a prisoner and you do have choices! 