[[WMG:Carol is a clone of Mike]]
Some crazy mad scientists were working on a way to artifically create HalfIdenticalTwins, and they paid Jason and Maggie for permission to experiment on the newborn Mike. Maggie was employed as the human incubator for said experiment, to cover up what was goin on. The experiment worked, sort of. While Carol didn't turn out quite as identical to Mike as they hoped, she was remarkably close. Afterward, the scientists moved on, and nobody in the Seaver clan spoke of the incident ever again.
* Or, being siblings they just look a lot alike.
[[WMG:Ed and Irma previously had an affair.]]
Based on the way they behaved in the episode Anniversary from Hell, I don't think it would be a stretch to assume they'd been involved in the past. Most in laws, even ones that have been related by marriage for decades, don't greet each other like old lovers. We also see Kate and Irma have a mutual dislike of each other.