I'm Canadian & not overly familiar with US history & until I read Twelve Years a Slave, I had no idea anyone ever used to own slaves in the north. I'm doing a bit of genealogical research and have traced some of my United Empire Loyalist family back to New York State in the mid to late 1700's. Just ordinary land-owning farmers, but what are the odds they would have been slave owners?
The odds were low, but not impossible. New York had the largest slave population of any of the states which eventually became free states.
In 1790, the black population of New York State was 25,875, of which, 21,193 were slaves.
Source: http://newyorkslavery.blogspot.com/
In 1799 New York State passed a law providing for the gradual emancipation of slaves. All slaves eventually became free in New York State in 1827.
The total population of New York State in 1790 was 340, 120. So, slaves were 6% of the population.
Source: http://www.infoplease.com/ipa/A0004986.html
It is not impossible that your family were slave owners, but probably not likely.