Spoke nipple broke, should I replace more than one? I had a spoke nipple break on my rear wheel, the threaded part broke off right where it enters the wheel.

The nipples appear to be aluminum alloy, which I believe are not as durable as brass nipples.

Since I have to replace the failed nipple anyway, is it likely that others were overly stressed after the first one failed and should I replace more than one (or all of them)? Is there any issue with having some brass nipples and some aluminum? 

It's a rear wheel with 32 spoke, 3 cross lacing.
I would just replace the single nipple that is broken with a new brass nipple. It sounds like these wheels are not very expensive so I wouldn't want to invest a whole lot more time and money in replacing something that may or may not be a problem in the future.