Sister Mom is the first part of the 23rd episode of the second season in Dexter's Laboratory. In this episode, Dexter has to attend a parent-teacher conference regarding his grades, but instead of having Mom attend, he has Dee Dee take her place.
Dexter and Dee Dee are walking home from school for the day. Dexter has a note in his hand regarding a parent-principal conference. Annoyed, Dexter hatches a plan with Dee Dee to prevent Mom from finding out about the conference, by using the Molecular Makeover Machine to change Dee Dee's outer appearance into Mom's. At Huber Elementary, Dee Dee and Dexter are waiting out in the hallway for their principal to see them. During the meeting, Dee Dee starts behaving erratically and causes interruptions. The principal tells her that Dexter got an A minus on his last test. Dee Dee then yells at Dexter for something so trivial while Dexter yells that it's Dee Dee's fault he got an A minus, which causes a fight between the two. The principal ends this by making both Dexter and Dee Dee attend a meeting with their school counselor. During the meeting, Dexter, disguised as Dee Dee, makes her sister look like an idiot while Dee Dee, still disguised as Mom, tries to defend her intelligence. A dog, disguised as Dexter, barks and pants when asked a question. The counselor ends the meeting early. She and the principal go get lunch while Dexter and Dee Dee argue.
There is no need for a Parent-Teacher Conference if Dexter got an A minus in his previous test. Though it's likely a big deal since Dexter is a straight A+ student.
When Dexter activates his makeover machine he says "It's Morphin' Time!" while doing a fist in hand gesture. This is a reference to the morphing sequence in the original Power Rangers television series.