OP here (I didn't hook my email when I orginally submitted)
To clarify, my aunt and uncle were married with kids before they became step-sister and brother. My grandmother died (my uncle's mother), and coincidentially my aunt's mother had been widowed for some time. My grandfather (my uncle's father) married my aunt's mother. I just still think it is hella weird, but we just joke about it when we meet around for the holidays.