Is the sigmoidoscopy a risky thing to do?

Not generally. Flexible sig: getting cleaned out by enemas, having a flexible, lit up tube inserted into the rectum, and advanced (steered), under direct vision (video chip inside the scope) from rectum to sigmoid, and descending colon; looks at last 1.5 feet (approx) of intestine. Stops at the turn under the left ribs. No sedation usually, can do biopsy; slight risk of bleed, perforation (1 in several thousand.
Not really. As with any medical procedure, there is some risk, but if you are a relatively healthy person, it is very safe. There is a less than one in a thousand risk for the most common complications, which are bleeding, perforation of the bowel and infection, but if you have a well-trained specialist perform the procedure, it is extremely safe.