Edwina was a former lover of Abraham Simpson, whom she met in England during World War II. There relationship was good and eventually became a sexual one. Abe unknowingly impregnated Edwina, and left England. Edwina then gave birth to Abbie Simpson nine months later, Abe's first child. Abbie is Homer's and Herbert Powell's half-sister.
Some odd 60 years later, Edwina and Abe met again, introducing her daughter to Abe and the Simpson family for the first time.
Supposedly, when Abe met Edwina in the war, he led her to believe he was shipping out in the morning, not knowing he actually was shipping out. Unbeknownst to him, he impregnated her, not knowing until he met her while leaving London. Edwina is also the reason why Abe wanted to go to London in the first place.