First, the first two characters said they were stranded from a broken car. If so they were near a road or highway. So why didnt either of them think to go to the highway and get help or hitch a ride?
* spoiler alert *
Second, if the three were all related and parents of each other, then they would have recognized each other from photos.
First, as can be seen in the movie, both Tom and Samanthas cars where stranded on a small forest road (in the 1940's mind you) so there wouldn't be a lot of traffic, or you know.. any.. Also, every time they tried to move too far away they simply ended up back at the cabin.
Second, Jody was raised by Samanthas husbands parents, which we gathered hated Samantha, so they were probably not keeping a lot of photographs of her laying around. Tom was raised in an orphanage and the only thing he "inherited" from Jody was a small box of junk she had on her when she was arrested (including the necklace with the picture of Samanthas mother), it is unlikely she would have had photographs of herself on her at the time of the arrest.
So the only person that might have recognized her parent would have been Samantha if she had seen enough pictures of Hans, but i'll buy that maybe she only had seen a handfull of photos a long time ago which might not be enough to notice that it was her father, hell I would even buy that she had not seen any picture of him at all, that's not exactly a stretch..
All of this is explained in the movie.