Q: Can I backup my wife's iPhone to my icloud

Hi djhak,

Thanks for the question. The short answer is yes - you could do this. However, some important information may be excluded from the backup (like her iCloud account settings, and any content that is not kept on the device when signing out of iCloud). The process might also be cumbersome if you are planning to do it often, as constantly switching iCloud accounts could potentially merge your data or result in loss of data.

Have you thought about backing up her iPhone with a computer with iTunes instead?

Should I use iTunes or iCloud to back up my iPhone, iPad, or iPod touch? - Apple Support

http://support.apple.com/en-us/HT204136

Or, of course, purchasing iCloud storage for her account?:

iCloud storage upgrades and downgrades - Apple Support

http://support.apple.com/en-us/HT201318

- Matt M.