They end up deleting each other's Contacts and Calendar events, perhaps getting each other’s messages and photos, and often have run out of space in iCloud. Unless you really do want to share your calendar, contacts, notes, reminders, photos, messages, etc with someone else, do not use the same Apple ID for iCloud as any other person because
iCloud accounts are not meant to be shared!
It is OK to have an iTunes and App Store account (Apple ID) that you share with others in your family - this will save you having to purchase music, movies, books, apps, etc multiple times. But iCloud is different. You should have your very own Apple ID for iCloud - which can be an email address that belongs to you (and is not used for iCloud by anyone else), or can be a free Apple iCloud email address that you can easily set up.
iCloud accounts are designed to be associated with one person - to allow that person to sync their important data (contacts, calendar entries, notes, reminders, photos) between their devices and, if desired, to back up each Apple mobile device belonging to that person.
I like to picture an Apple shaped cloud floating above me wherever it go. This cloud is labelled with my email address to identify it as mine and mine alone. My husband Jim and son Jacob have their own iCloud clouds, since I don't want their data to be mixed up with mine.
When I do things on one device - for example, add a calendar event or contact, or take a photo - that 'thing' will appear magically on my other devices (but not on their devices).
Not only that, my iCloud cloud will hold on to this information - so if a major act of God leaves me with no iPad, iPhone or computer, my information is still safely floating up there in my cloud - waiting for me to get a new device. The minute I tell that new device about my 'cloud', all of the important information that is sync'd to my iCloud will downloaded onto my new device. And because I also back up my iPad and iPhone to my iCloud, I will can restore all my photos, messages and app data to the new device - ready to pick up where I left off.
Just be careful when you try to unravel any existing iCloud tangle, as you may end up unintentionally removing your own important information from your iPhone or iPad.
iTandCoffee can help sort out any iCloud mess in your family. Just call 1300 885 420.