I have some people like this in my personal life and I handle it by using the accoutrements of a business meeting. Chief among these is some sort of shared document. It might be a google doc you can both edit while you talk on the phone. It might be a piece of paper that sits on the table you are both sitting at. It might be a whiteboard or a flip chart. The tech isn't important. What it is important is that you can both read it any time and write to it any time.
At first, it's an agenda. Typically in meetings that have a history of going off-plan and wandering, you also have two areas for things that come up during the meeting: New Business (we will discuss this meeting) and Parking Lot (we will not discuss this meeting but don't want to lose the idea.)
As the meeting proceeds, it sort of becomes minutes. Someone writes "4 hours Sat morn" next to an item, "Blue" next to another and "call Tuesday" next to a third. This reduces the chances they will forget you settled something. If your family member loops back and starts saying they want to do a particular thing in the afternoon as well as or instead of the morning, you don't have to start from scratch on that topic. I might respond, for example, "do you think it might take more than 4 hours, or is there something else you'd like to do in the morning that could push this to the afternoon?"
(There is a kind of conversational technique I see a lot with people of a certain age where they pre-rebut your arguments against something. They may want to suggest doing a different thing in the morning, but expect you to point out the morning is full, so they get you to agree that actually that thing could be done in the afternoon, and if you do then presto, they've made room for their preferred task.)
Asking "are you coming back to the thing I thought was settled for [reason] or [reason]?" is generally less argumentative than "why are you coming back to the thing I thought was settled?" because many people hear "why are you X?" as "I think you should not X" no matter how many times you tell them you mean what you say quite literally.
If the issue is that they want to know exactly how many balloons need to be ordered, and you're not planning to order balloons for two months yet, you put "determine # balloons needed" in the parking lot. Maybe with a date on it about when it needs to be done by.
The less of their suggestions you reject or ignore, the happier they will be. If this is your event, you're running it, and they don't get a vote, it's still nice to make them happy. If they do in some way get a vote, perhaps because they are paying for it or hosting it, then it's even more important they feel listened to. But I give you this advice not to help them, but to help you. Clearly, when this person doesn't feel listened to, they make their point again. The more you hear them, the less they will circle back, repeat, drill too deep, and so on. The world of "effective meetings and planning" has a lot to offer you here.