Does anyone know of a good source on Primitive Portraiture

Back in 2009 I bought this large portrait of two sisters at Auction and have always thought it so interesting. The seller had a little info on the subjects, sisters Lucy and Mary Collins from New York. So many of these primatives are unsigned but I am pretty sure it dates from the 1820-1850 period and looks like a few well known primitive artists of that period. I am trying to find someone who knows this type of portraiture. Anyone have any ideas? Thanks!

 

I wouldn't call this a primitive or folk art. It's a bit more refined.

What does the back of the canvas look like? Have you removed it from the frame to see if the signature is underneath?