I go to the homes of relatives on a daily basis, can I use the term at their home? Thank you.
ただいま is typically used when returning home (though can be used casually say when returning to my desk at the office).
If you go to the relatives often enough and it feels like a "home" then I don't see any reason you can't use it.
(Rather than asking us, why not just try saying when you get there next time and see if you get an おかえり back?)