As a person myself who hates vegetables but is being forced to eat them in order to correct nutritional imbalances in my body, I can symphasize with your sister-in-law. My naturopathic doctor wants me to eat seven to nine servings of fruits/vegetables/day. Say what? LOL I'm lucky if I eat ONE serving.
I was eating salads with lots of raw veges until I grew tired of eating salads. Then I happened to be in Costco one day and saw a lady demonstrating the Vitamix machine. Wow, did that make a difference! I can now "drink" 3-4, sometimes 5, cups of veges/fruit/day for breakfast and not get sick. And I still have lunch and supper to get "the rest" of the veges/fruits in. Sometimes, I'll make two smoothies a day.
My breakfast smoothie consists of (everything is organic): blueberries, blackberries, strawberries, raspberries, summer squash (good for potassium; I can't eat bananas), zucchini, carrot (w/top green), celery, kale (spinach is also good; I can't eat spinach), cilantro or Italian parsley (a good diuretic, if one retains water), and cabbage (a wonderful product for fiber and regularity; hint: it helps with constipation); I also add ground flaxseed and a tsp of Raw Hanuka Honey.
You only need a little of everything. Once all of this is added to about 1-1.25 cup of water and everything is blended together (at the rate of about 240 mph), you have a smoothie or a gelato (mine is usually a gelato, because I freeze all of my veges and fruits).
Granted, the VitaMix machine is quite expensive, but it was the best investment I ever made for my health.
And now, I'm going up to make myself my morning smoothie/gelato!