how do I get someone to remove their belongings from my property

There is not much in the way of "black letter law" as to such matters.  Of course, if there was some formal agreement, bailment, contract, etc., between you and your relative, the terms of that agreement would be pretty important.  In the more normal situation of a "he said/she said" as to what either of you said or intended a year ago, I can express very little of an opinion.  The way this usually plays out is a disagreement as to what those terms and conditions were. I would send written notice that the stuff has to be removed, and give a due date (keep a copy of the letter), informing the relative that if it isn't, you will be depositing it into a storage locker at XXX place, at his/her expense (ditto on the moving costs, if you have to hire someone).  Then have it moved, put the storage unit in your relative's name, and tell him or her how to get the key to the lock.