"Sesame Street" - Bert and Ernie Quiz

Answer: sub-basement

They have a sub-basement apartment in the 123 Sesame Street building. You can see their criss-cross windows at ground level.

Answer: same bedroom, two beds

Bert and Ernie share the same bedroom, but each have their own beds with their first initials carved into the headboards. They have a night stand between them, often with a lamp on it.

There are skits out there where Ernie is in his own room, but the backstory there is it's so messy that he sleeps in Bert's room.

Answer: pigeon

Bert loves pigeons and has a few of them as pets. His favorite one is Bernice, which he often talks about and is often seen in skits.

Answer: oatmeal

Plain oatmeal is his all time favorite. In fact it's the only thing he loves as much or more than pigeons.

Answer: eats the excessive amount

There are two famous skits showing this:

The time they are to have a lunch of, specifically, "cheese, sausage, anchovy, and pepperoni pizza" and a glass of grape juice, but Ernie clearly had more of each than Bert, so he solved it by eating and drinking his portion until he had less, then he started eating and drinking Bert's portion until Bert walked away disgusted.

The other time was similar. Ernie and Bert were to have an afternoon snack of cherries, of which Ernie had each portion on either end of a balance scale. But rather than move the excess cherries over to the other side, he kept eating the cherries until they were down to just one each, at this point Bert screamed and fainted, so Ernie just assumed he wasn't hungry and ate the last two.

Answer: True

In one skit, Bert came home to Ernie on the phone with Grover and showed that he got a new phone and number because Ernie is on the original phone all the time. After understanding Bert's logic, Ernie then made a call... to Bert's new phone, and talked to him through there, stating how they could call each other all the time and talk to each other that way. Bert ended up fainting from this, as it wasn't what he imagined.

Answer: 3

The three relatives Bert often spoke of or we saw were as follows:
His twin brother Bart, who had a very opposite personality to Bert, as shown when he came to visit Bert and Ernie in one skit.
His nephew Brad (presumably Bart's son) who was over at the house and Bert was about to give a bath to when Ernie chastised him for not having any bath toys, then proceeded to put too many toys in the bathtub.
His Aunt Matilda, whom has never been seen, but Bert is often speaking of for various things, like what to get for her birthday or when Ernie opened a present that was hers thinking it was for him.

Answer: Yes

It happened only once. During a skit where Bert's plate of cookies was ransacked and he accused Ernie of eating his cookies and even described the culprit as having black hair and the same striped shirt as Ernie. However Ernie denied having anything to do with it, but then Cookie Monster dressed like Ernie, took Ernie's plate, said "Hi Bert" then laughed like Ernie, and walked away. Ernie and Bert, both shocked at what they saw, both fainted at the same time.

Answer: Rubber Duckie

Rubber Duckie is Ernie's very favorite bath toy, either in or out of the bathtub. Without him, his baths are just not the same.

Answer: Ernestine

Ernestine, what else?
She was shown in a skit where Ernie compares himself to her, and was also shown in the video, "The Best of Ernie and Bert".