Ivor's relationship

Yep.

Yep. I think so - half-brother most probably. His mother was the second (polygamous) wife of Lars. There's a discussion with his mother early on where he refers to himself as the bastard son of Lars, and she explains at the time they were married and Eric born that polygamy hadn't been outlawed. Utah only became a state in 1896 after outlawing polygamy and giving women the vote. Eric was wounded in WWI so he's probably mid 20s, maybe approaching 30 when the movie takes place - born a few or more years before Utah statehood.