One of the strangest family trees I've personally seen was the one that dealt with the relationships between the Biblical patriarchs and matriarchs and their various family members. I was researching them for my Comparative Religion class and my mom walked in as I put a chart of the family tree on a slide. The conversation went something like this:
Mom: What's that?
Me: Oh, I'm doing a report on the Patriarchs and Matriarchs. This shows who they all married.
Mom (examining the chart): Did that one marry her uncle
Me: Looks like it. Do you see Jacob down there?
Me: He had two wives. Each wife also gave him their maids to give him more heirs, so he had four women.
Mom (looking closer at the chart): Holy s#it, the maids were his wives' half sisters!
Me (looking closer): Wow, you're right!
Mom: This reminds me of that song "I Am My Own Grandpa"
I'm sure there are stranger family trees, but it was pretty funny when I looked closer at it.