Probably best to go the way Groom suggests.
I had a similar situation. My father's youngest sister turned out to be his niece, i.e. my cousin.
This I discovered when I got her birth certificate, with the aim then of finding out ages of all her older siblings.
She had three children, two girls one guy.
Her son's wife was interested in FH and asked for info on who she thought were her husband's grandparents [his mum's 'parents' as listed on her marriage certificate]. After chewing my fingernails a lot, we let her know the full story and left it to her to tell him and his sisters who were at that stage all in their 30s.
Result: Son's reaction "Well, isn't that interesting!"
Older sister's reaction "I don't believe it" and she no longer talks to anyone in the family including her brother and sister
Younger sister's reaction "Doesn't worry me, she's still my mum!"
DawnM
