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 because she keeps bothering him, 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 normally a straight A+ student.
The title card shoes Dee Dee, back to normal but dressed as her mother, reprimanding her brother for writing the episode's title on the wall. This indicates that Dee Dee can be firm to her brother if necessary, especially when he does something careless.
For plot purposes, Mom was slightly out of character in this episode, being overly complimentary to her children, praising them and giving them tons of compliments.
When Dexter activates his makeover machine he says "It's Morphin' Time!" while doing a fist in hand gesture similar to the pose that Jason and the other Rangers use when getting ready to morph. This is a reference to the morphing sequence in the original Power Rangers television series, Mighty Morphin Power Rangers as well as Power Rangers Zeo.
First on-screen appearance of the school Principal, and second appearance after a "voice only" cameo in "Dexter's Rival".
The Dog's makes an out-of-body cameo in this episode, having switched bodies with Dexter at the end. His original body was never seen, but his voice was heard and his mind was embodying Dexter at the end.
It's implied a girl at Huber Elementary, and likely her brother, have an abusive father. In the waiting room for the parent/teacher conference, a girl with a black eye was sitting next to her angry looking father. In the waiting room, Dexter would punch Dee Dee (who was disguised as his mom at the time). The parents in this family looked at them in discontent (especially the father), while the kids in the family were happy and gave him an excited thumbs up for, presumably standing up to his abusive parent. Later on in the episode, a child's drawing was seen in the school therapist's office, depicting an angry stick figure attacking a house, titled "Daddy". This could have been drawn either by one of the kids from the aforementioned family, or by another kid with an abusive father. Given how nonchalant the therapist was about Dexter's comically dysfunctional family and how she had the picture hanging on her wall, along with an innocent drawing of a rainbow, the school doesn't see anything wrong with this abusive family's dynamic, and is taking no action to stop it.
Apparently, Dexter and Dee Dee were walking home from school, without backpacks. The only thing, being taken with them was Dexter's note.
When Dexter was crying and running up the stairs, the note from school had completely disappeared. Yet, in the next shot of him entering his room, it was in his hand again.