Although Apple recommends creating a backup of your iPhone before installing iOS updates, you can install the latest system updates for your phone without a backup. … It merely provides an option to retain previously saved content such as contacts and media files in case your iPhone runs into problems.

Is backup necessary for IOS update?

It is RECOMMENDED that you backup prior to any updates but not required. I have updated plenty of times without backing up and have been fine. Your sister’s iCloud won’t make a difference. Once a device’s software and firmware are updated, they remain that way, regardless of which iCloud is logged in.

Can you update IOS without iCloud?

iCloud has nothing to do with upgrading your device. You need to be signed into the iTunes & App Store, but you do not need to be signed into iCloud. You also need Wifi if you are going to upgrade OTA. Otherwise, you can hook your device up to a computer that is running iTunes, and update it from there.