Melinda (her face is seen on Chas's computer as he was making the Finster Family tree)
As Chas starts the Finster's family tree, Tommy and Chuckie decide that they're brothers. Meanwhile, we learn that a new branch of the family is on its way when Didi announces that she is pregnant. - Description from Klasky Csupo.
Stu and Didi go on a cruise to Mexico for their fifth anniversary, so the Finsters are left to babysit Tommy. Chas pampers Tommy because he was playing alone and Charles thought that meant he was lonely. Tommy and Chuckie see a family tree and thought babies grew on trees. Then, due to sharing some interest, Tommy and Chuckie decided they must be brothers. Meanwhile, Didi is vomiting due to nausea, presumed to be seasickness, but it turned out to be morning sickness as she is pregnant.
This episode marked the first appearance of Chuckie's paternal grandparents (Chas's parents), Shirley and Marvin.
Marvin and Shirley are said to still be in their fifties, so they're significantly younger than the rest of the grandparents on the show, with Grandpa Lou being in his late 60s, and Didi's parents presumably around the same age as Lou. It can be assumed that they were probably somewhere in their twenties when they had Chas.
It is revealed Stu and Didi have been married for 5 years, meaning they were married for about 4 years when Tommy was born.
It's possible they also have been married for the least amount of time compared to their friends.
The title card sequence is used twice in this episode -- once after the opening credits, where the title card says, "The Family Tree Part I," and again after the first commercial break, where it says "The Family Tree Part II." Also, the music in the second title card is different than the regular fanfare used in the first title.
Since Stu planned to go on the cruise ship to Mexico over the weekend, he should have planned to have someone watch Tommy in advance.
Cruise ships can't go to El Pantalón, Mexico, because it's nowhere near the sea.
Throughout their anniversary cruise vacation, Didi becomes very ill, with her and Stu assuming it's seasickness. Morning sickness (what Didi was experiencing), happens during the first trimester of pregnancy. This starts during the first couple of weeks and ends after a month of being pregnant. Stu even confirms this in the final scene when Didi starts to become sick in the doctor's office, "We have to get used to this again".
There is never a scene shown of Didi vomiting, as in the classic episodes, there were two instances that we actually get to see throwing up in action--when Tommy threw up on Angelica in Slumber Party in his sleep, and when Angelica threw up on Drew in Chuckie Loses His Glasses. Apparently, tastefulness is not the problem here, as Nick continues to show those two episodes, uncensored so it is unknown why. On the other hand, one of the creators, Arlene Klasky, doesn't go for that sort of thing, according to a recent article in The New Yorker. Dil also spits milk up on Stu in Accidents Happen, and in The Way Things Work (Dil spits up on Tommy and Tommy does call it an oopsie) uncensored, and censored throwing up was shown in A Dose of Dil (by Chuckie due to sensory nausea), Rugrats Go Wild (by Kira due to seasickness) and Ransom of Cynthia (due to being spun by Drew, by Angelica.) Throwing up is mentioned in The Rugrats Movie (Chuckie in the truck), Raising Dil (Stu remembers Dil spitting upon his cake), Tommy and the Secret Club (Chuckie mentions barfing in his poem), A Very McNulty Birthday (Angelica suggests lying to Colleen McNulty that Timmy McNulty threw up so he won't get cake), The Age of Aquarium (Chuckie remembers his dad throwing up on a boat) and Accidents Happen (Chas and Chuckie both throw up on the roller coaster).
But, with Rugrats not being a total "gross-out" show, things like vomit could be unseen to not gross out the audience. It can also be seen as unnecessary
One of Angelica's "bad" tactics was to have Chuckie spray water from a gardening hose onto Tommy. However, water was sprayed everywhere in the room that Tommy was in, and in that room was the computer that held the family tree. For some reason, the computer escaped the watery mess.
This episode also served as a sort of pilot to The Rugrats Movie, which was released in theaters on November 20, 1998. However, the only link between this episode and the movie is at the very end, when it's revealed that Didi is pregnant. Other than that, it's more or less a normal Rugrats episode.
This is the last episode to air in 1998.
This is also the last episode to use the 1991 Klasky Csupo logo after the credits. For The Rugrats Movie and the later episodes onward, the 1998 Klasky Csupo logo is used after the credits.
The adults sing "Happy Anniversary" to the tune of William Tell Overture during the episode's opening.
There was no significance towards who was looking after Spike.
Chas is said to be allergic to dogs and cats, so it’s possible Spike was put in a kennel or let out/fed by a neighbor