Sims 3 works fine in my old Mac Pro under both Mavericks or Yosemite. So, New OSX is not a problem at this moment.
But, in Yosemite, one of the graphic setting may cause issues, most likely driver's problem. So, I will say it's possible to run Sims 3 in your iMac, but you have to find out the optimum graphic settings for difference OSX.
However, I am sure you won't expect high performance with those hardware.