Sinclair is one of the oldest surnames in Illinois. One Sinclair came with the Pruitt family from Virginia, North Carolina and Tennessee area in 1806.
From what I recall someone who is of Italian descent the Italians first went to the larger cities such as New York, Boston, Philadelphia, New Orleans and St Louis and if the area got too big then they ventured into smaller communities in the general area. Most Italians didn't start coming over until the mid-1800's.
Unless the surname was changed to Sinclair from another surname, I don't see how anyone in the family could say it was Italian. However, my father said my maiden name was German but from what I can find I'm not sure about that but do know that his paternal grandmother's parents were both German immigrants and perhaps he heard of the German connection and didn't hear which family was being discussed.
Debbi Geer