John Emery wrote:
SNIP
I'm wondering if anyone has ever heard of beach sand getting in the
drain pipes of a house. How could it get there? Is it backing up from
the city sewer, or coming in from the vents? If from the vents, which
are simply open at the top, is there anything I can do to stop the sand
from blowing in? Could the sand be the cause of my drain problems?

Better chance is that there is a fracture in the line underground.
That can allow soil/sand to wash back into the pipe over time.
A snake is usually ineffective with this kind of clog as the sand
simply closes back over the snake. The ShopVac may be better
as is pressure sewer cleaning equip.

Try and get it cleaned for now, but you may wish to have the line
video taped to look for the cause.

As suggested, ask the city for any relief/advice they might offer.

Jim