Hello Scott,
I purchased a large formal portrait of an elderly woman in about 1980 in a 2nd hand shop on Martha's Vineyard. It is signed Olga Sorensen and the signature looks exactly like the signature on Indian Steve's painting. The signature is oddly enough on the upper left hand side of the painting, not the lower right which is usually the case. It doesn't sound like she did many portraits and I wonder if you have any further information about your great aunt in the meantime. I moved to Germany in the late 1980's and this fine woman is hanging in my front hall. I actually wondered if it was a portrait of Olga Sorensen when I bought it all those years ago.
I can send a picture if you are interested. I don't know if you are still active on this forum since this was several years ago.
Regards,Tina