Design question on quick Project

[Steve N]

Type of, and diameter of pipe can be manipulated enough that the span your Sister seeks is easily doable. You are looking at 1 1/2" thick shelves, not a lot of sag happening there. If you have concerns you could always add a hardwood nose piece as shown below. With this either ply, single or double thick or solid wood can be made much stronger, and with much less flex.

[Alan S]

The shelf thickness sounds fine for a non-sagging light duty shelf of that length.  The bigger problem is distortion during drying.  My favored approach, probably not applicable here, is to buy more dimensional lumber than I'll need for such a project, and store it for a few months to let it dry.  The pieces that remain straight and untwisted are used for long parts, and the stock that has warped gets cut up for short parts.  Even twisted stock that has been cut narrower and jointed and planed usually behaves well.  If it's warped enough though, the parts fit in the fireplace.

[AgGEM]

Solid core door(s). Rip to widths.
Edge band it.
1 & 3/4 thick, you can place the pipes anywhere you desire. Be plenty strong.
Fast, cheap, strong.
Pleasing....
Ag