""And if you don't know you are biologically related to someone, you may become attracted to them and tragedies like this may occur.""
The former Liberal Democrat MP raised the couple's case during a House of Lords debate on the Human Fertility and Embryology Bill in December.
"But when we are unaware of that relationship, we are naturally drawn to people who are quite similar to ourselves.
"And of course there is unlikely to be anyone more similar to any individual than their sibling."