Hey there!! First of all....it's great that you've found a place where you and your sister can connect with folks who "get it". Secondly, don't panic....this is day by day, learn as go!
I wanted to suggest that you can greatly reduce sugar in baking etc., by adding more spices. For example, in making muffins, I reduce the sugar demands in the recipe by over half, and add a lot of cinnamon. The taste of cinnamon fools you into thinking that it is sugary
For beverages, skip the straight up juices, skip the pop, careful with milk...it contains lactose, anything ending in "ose" is sugar. Read the nutrition labels on the side of packets/containers. Water is great, of course.....but it gets boring. What I do is flavour the water....but not artificially. I add fruit combos, or fruit/veg. combos to my water. Say, strawberry with melon, or strawberry with mint leaves....or sliced cucumber with raspberries. The sky's the limit. Anyway...experiment. It's all according to individual tastes. Add to that walking. It will benefit you in a myriad of ways. Last but not least....stay connected. Hugs....you've got this!!