When is it wrong to fall in love

the question is

is it right or wrong to fall in love with a distant relative of my wife or not

the story

when me and my wife got married this girl was only young so we did not really know each other, but when she got married there was some what of a spark between us. however that was a few years ago

the dilemma

her husband left her for someone else and she came to visit us while she was here the same spark was very evident and things developed into a full sexual relationship.

so back to the question is it right or wrong to fall in love with her?

comments welcome please