Annoyoing, combative sister-in-law?

My husband's brother's wife has got to be THE MOST annoying personality I've ever run across. I don't see much of her, but when I do, she always manages to get under my skin. She's a judgemental religious fanatic and know-it-all. She's two-faced and is good friends with my husband's ex-wife (mostly to spite my husband and be nosey). Anyway, no matter what I say,(even when I'm not speaking directly to her) she has to retort. I said to my father in law "Amish people don't use electricity". She says "Actually, they do". I say "I don't think my husband will lose his hair". She says "Actually, he still can". When I was pregnant, I said to my mother in law "It's a girl"! She says "I'm going to tell you why what your doctor said isn't accurate". She's a very miserable, negative, just all around unhappy individual. And it grates on my nerves. I don't know my in-laws well enough to handle it in my usual sarcastic manner, plus they're hyper-religious and conservative. Any advice on how to deal?