Intertwined Family Tree

Researching my family tree has had some interesting curves thrown at me.  While following a tree up (or is it down?) the other day on Ancestry.com I found  one of my ancestors, Thomas Alexander, born in 1548, in  Ayrshire, Scotland.  His parents are listed as Thomas Caldwell and  Alice Tomlinson .  He married Elise Muir, born 1558 in Lanarkshire, Scotland.  her parents were Robert Muir and Alice Tomlinson.

Alice Tomlinson?  That is the name of her husband Thomas Alexander Caldwell's mother?  So, going a notch higher I found that Alice Tomlinson  was married to Thomas Caldwell and later married to Robert Muir.

Thomas Alexander Caldwell and his wife Elise Muir had the same mother.  Thomas married his half sister! 

I thought Ray Stephens made all that stuff up!