THOMASTON, Maine — It’s not every day that you find an original painting by Andrew Wyeth in a relative’s house that you’re helping clean out. But if your aunt was Christina Olson — depicted by Wyeth in his iconic painting “Christina’s World” — your odds improve. That’s how John Olson Sr. came in possession of a 1942 mixed media painting of the Olson House in Cushing by Andrew Wyeth that the painter gave to Christina six years before the artist finished “Christina’s World,” according to Kaja Veilleux, founder of Thomaston Place Auction Galleries. The painting is one of three works by Andrew Wyeth going up for sale at Thomaston Place Auction Galleries’ summer auction this weekend.