The sigmoid colon is the lower part of the colon...I have crohn's (crohn's colitis specifically which is when CD is affecting the colon) but my mom has UC so with them both being an IBD the fact that your dad had crohn's doesn't mean that you can't have UC or that you'd only be at risk for getting crohn's and not UC. Of course one can easily be misDX for the other so make sure you find out what the pattern of inflammation is in your sigmoid, if it's patchy (skipped patterns of inflammation with some healthy tissues in between inflammed) then that would be crohn's, if the entire area is inflammed with no skipped patterns of inflammation then that would be UC...also another difference in the appearance between the 2 is with UC the inflammation would be on the surface of the lining only, with CD the inflammation can go through the many layers of the intestinal lining (but doesn't necessarily always).
There are many UCers that also have reflux issues so that doesn't necessarily mean you more likely have CD either.
My bum is broken....there's a big crack down the middle of it! LOL :)