D.C.
Switching over to sippy cups means that most of her nutrition is coming from food now. The sippy cup is not her main source of nutrition like the bottle was, so she won't take the same amount of milk as formula or breastmilk. When I weaned both of my babies, I started with the daytime bottles. Instead of giving a bottle before or after breakfast, I'd give a cup of milk with breakfast. After a few days, I gave a cup of milk with lunch instead of a bottle afterwards. Then, the same with dinner. I just switched all the mealtime bottles over to a cup of milk with the meal. After a couple weeks, I started offering a cup of milk to drink while we read bedtime or naptime stories. (They drink some, but not 8oz like they took from a bottle. It didn't interrupt their sleep at all.) Neither of my kids seemed to notice or care. As long as they didn't see the bottle, they didn't notice it was gone. After a week or so with no bottles, they didn't care at all, even if they did see other babies with the same kind of bottle they used to drink.
Good luck. It's a process, so don't worry too much. Just start with the daytime bottles and see how she does. Then, after a couple weeks, switch the bedtime bottle. She'll be fine. I was worried, but it was actually much easier than I imagined.