Why does my sister's fan belts keep breaking.

That's it? No other details? Well, belt was too tight, pulleys are out-of-alignment, generator or power steering pump is locking up, belt is rubbing on something, belt is too loose and flopping around, ...

Are the broken belts shredded or is there just one clean break? Has an experienced mechanic ever looked at one of the broken belts? Are you installing automotive belts or one from a hardware store meant for a wash machine-type of application? Which engine size do you have?