ominous vs sinister

ominous and sinister both are adjectives.

Word	Noun	Adjective	Verb	Adverb
ominous	No	Yes	No	No
sinister	No	Yes	No	No

As adjectives, ominous and sinister are synonyms defined as:

ominous and sinister: threatening or foreshadowing evil or tragic developments

Other synonyms of ominous include baleful, forbidding, menacing, minacious, minatory, threatening.

ominous (adjective)	sinister (adjective)
threatening or foreshadowing evil or tragic developments	threatening or foreshadowing evil or tragic developments
presaging ill fortune	stemming from evil characteristics or forces; wicked or dishonorable
	on or starting from the wearer's left