MD to Myrtle beach by way of Monticello?
 

Starting in CT
We have to attend a family function in southern MD.
After function we thought we’d spend a day with relatives, then (and this is the question) drive to Myrtle Beach to visit another brother.
Spend a day.
Drive to Monticello
spend a day.
Drive back to CT
Is this too much? (I think it is, DH doesn’t)
Should we spend more than 1 day at each destination? We only have 1 week

It would be too much for me especially the long slog to MB for a 24 hour turnaround. I would pick one place and spend a couple of nights there.

Yes, too much for just a day in MB, but if you haven't seen the brother in a while, and won't again, give up Monticello. OR ask the brother to head up the road a bit and meet there.
Add some days.

Assume you already know but the distance from a place such as LaPlata to MB is a little over 400 miles one way and it's about the same distance from MB to Charlottesville. I agree with at least adding a few days somewhere.

If you just want to go to A beach I think you'd be better off going to the Outer Banks than MB but that's based on my own beach-related personal preferences.

Thanks.
That is what I thought.
We can fly down to MB some other time.