Children’s story about siblings who travel to an alternate universe where time runs slower

Question

Ok so I remember reading this story in the early 2000’s, but it probably came out before then. Basically what I can recall is that it focuses on a brother and sister who found the entrance to another dimension I BELIEVE inside a cupboard or something. They travel there and meet its inhabitants and strike up friendships. However, time operates differently in this dimension, a couple minutes could equate to a couple hours back in their own dimension.

Eventually there is a conflict and the inhabitants ask for the siblings’ help to fight some sort of evil over there. What I think happens is that the brother stays and the sister goes back, and due to the time issue, when the brother returns so much time has passed that his parents/sister are dead, and the house is abandoned. I remember it especially because it was so sad, I think he then just lives out the rest of his life in the other dimension. Does anyone know the name of this?

I don’t remember the cover, and I’m sure it’s not The Boy Who Reversed Himself or a Wrinkle in Time.

Answer 1

Is this Marco's Millions (2001) by William Sleator...? It's a prequel to The Boxes (1998).

According to the Wikipedia page, a boy named Marco and his sister, Lilly, discover a portal to another dimension in the basement of their house. The insect-like inhabitants of that dimension are convinced that Marco and Lilly can save them from their god, a naked singularity. However, they find that for every one minute that passes in the other dimension, 21 pass on Earth, so Marco tells his parents that he's going to stay with a friend to explain his absence, which he expects to last a few days.

The protagonist is a boy named Marco who likes to travel. He often secretly rides buses far from home, though only his telepathic sister finds out. One day, his sister sees strange lights in the basement, and she and Marco investigate. They find a portal into another dimension, and thus the adventure begins. Marco finds strange insect-like creatures there, who are convinced that Marco/Lilly can save their dimension (and as a result save Earth) from their god, which is a naked singularity. None of their family members know about this portal, so Lily and Marco secretly need to save this dimension. Time is valued differently than on Earth in that universe, as 1 minute on the other dimension is roughly 21 minutes on Earth. Saving the other dimension would mean being missing for several days on Earth. So, Marco tells his parents he going to his friend Nat's for vacation, while he'll actually be in the other dimension.

According to this user review though, by the time Marco returns to Earth, he finds that over 20 years have passed and his parents and sister have died in a car crash.

Marcos's Millions is a great book about a brother and sister who have a inter-dimensional portal in their basement which leads to a world filled with bug like creatures who have been expecting him. And the only way he was able to pass through the tunnel was with a touch or hair strand of his sister Lilly who is able to see this world in her dreams and talk to the creatures to bypass a wall blocking the portal to the other world. In the other world time is normal but on earth it is really speed up. So Marcos is faced with many challenges on this world from trying to fix and repair a singularity that too is going to destroy the creatures and his world to try and be back at home and not go missing for a long time which ends up not going soo well when he goes missing in the other world of over 20 years where his parents and sister Lilly die in a car crash.