OP, with a big family reunion, I'd make it a Grand Gathering (or whatever they are calling it now) and go straight through Disney for this. They, I believe, have special capabilities for dining reservations, along with other offerings to your group that you can't get otherwise.
Yes, it does remind you that you have a ressie around the same time but it does give you the option to keep them both.
Yep yep. Absolutely!
Background:
We're having a big long trip, and 4 nights of it we're spending with family. I know NOTHING about what sorts of food they eat while at the parks (they are the long-time WDW-visitors, and I'm the newbie, but I'm providing the accomodations and I obviously play on the Dis while they don't, and I've already known things that they do not know, which makes me giggle a little bit). On the day that we check out of the 2 bedroom, first we thought family would be going home. Then we found out that they *might* be sticking around, staying at a hotel on their own, and we'd still meet up.
All of this was roiling around in my head, and then the 180 days came around. The family doesn't do the ADR thing, it seems, because no one wanted to talk about reservations with me. So I was making them mainly for the time that we won't be in the same villa with them; we'll just do counter service or whatever while we're with them. But on the day that we leave the 2 bedroom, I figured it would be fun to go to T-Rex. So I made an ADR for the 3 of us. Then realized that they might still be around, so I might as well include them. Since I didn't KNOW (and still don't), I made a second ADR for their group of 5. Same time.
It popped up a thing reminding me that I already had a reservation, and did I want to cancel it. I said No, and it let me continue. The T-Rex reservations are still there.
A few times I was changing times around slightly at the same resaurant and the same popup showed up. A few times I said that I'd cancel it, a few times I didn't, just to see, and it kept the two reservations when I did that. (then I would go back and cancel it separately)
So it might LOOK like it's telling you that you have to cancel, but it's not...
Anyway OP, I'd leave the TA and go straight through disney, at least for the disney part. Maybe for travel, if people need the help, stick with the TA.