Twincest is the 4th episode of the seventh season and caused great controversy, almost taken off the air.
Mary and Susan are spending a lot of time together, sneaking around and giggling. After hearing some suggestive sentences from a locked room ("Give it to me! Oh yeah, that feels good!") Johnny believes his sisters are in an incestuous relationship. He and Cory follow them around until they find out the twins are not in a sexual or romantic relationship and the thing Susan wanted Mary to "give to her" was a hand massager. They were sneaking around so much because they were planning a surprise swaggie party.
- The episode was controversial not for its sexual innuendo (this is a common occurence in Bee Shrek Test in the House) but for its incestuous suggestions.