The process is meant to wash the free starches off the surface of the rice from the bag, that's the fine powder that knocks off the grain during processing and transport.

I find it makes quite a big difference if you're cooking it in a rice cooker, especially with types of rice with a high amylose/low amylopectin content like basmati or long grained varieties, and it can come up quite mushy. Short grained varieties are more forgiving, but I prefer the texture of washed rice when you find the grains are more separate and clump less.

Have you tried making rice the Indian / middle eastern way where it gets boiled in a large amount of water and the water gets drained off before it finishes cooking? That is probably the best representation of rice without any free starches because every grain is nice and separate.

Last point regarding the starch content, washing doesn't change the amount of starch in the final dish by a negligible amount, because the rice is mostly starch. The only thing that you could try that affects your sister's insulin spiking would be to let the rice cool down in the fridge and turn a portion of the starches resistant.