Tangled Trees

Researching my family tree has had some interesting curves thrown at me. While following a tree up (or is it down?) tonight on Ancestry.com one of my ancestors is 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 parent s 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 married his half sister! I thought Ray Stephens made all that stuff up!