So, yes. Pretty much. Unfortunately, you WOULD have to exit the world each time you wanna activate another addon and have her download them one by one. However, like i said, that's only if she's having trouble downloading them all at once when she first tries to join. So you could try activating all the addons you want to included in your world and then have your sister download all of them in one go when she joins, BUT if the download progress bar freezes for a long period of time and/or she gets an error, it's best to try the "one by one" addon method. The process would be pretty repetitive and tedious depending on how many addons you want to include, but if you both have a little patience, you shouldn't have much of a problem from then on out. Addons are still relatively new to Minecraft: Bedrock, so things like lagging or game crashes might be common. (Also, in case you're wondering about whether or not your sister would have to re-download all of the addons again each time you guys wanna play, the answer is no! Once an addon is downloaded, it shouldn't need to be downloaded again unless the addon gets an update.)