In my state (Indiana), a divorce cannot be completed if the wife is pregnant. It's an item that must be disclosed in the petition for dissolution of marriage, at least as far as current laws are concerned. It may have been different during the time period you're dealing with, though. People also lie so that's another issue.

I'm assuming from your post that you are a subsequent child of your father, born after his marriage to Ms. Schwensen? If so, your father or some relative gave you the bare bones of the situation which gave you enough information to get more. I would think his wife did the same with your half sister as well, but you never know.

Have you checked to see if Ohio has an adoption registry? If an adoption did take place, it's possible your half sister might have registered as an adoptee, even if the adoption took place well before a registry was established. I wouldn't rule it out.

Also, check out some of the adoption registry websites online. Some of those are quite good and have articles about how to go about searching for people in adoption situations. Checking out other family history sites with surname boards is also a good idea.

Lastly, don't give up and think outside the box.