story identification - A book with two twins and a difficult relation, a room where time flows slower than the outside and a mysterious monster coming from a sink

There was this pocket book, back when I was a teenager (20 years ago), with these two twins discovering a room (it was something like a tool shed) where time flowed slower (1 hour inside was equal to 1 minute outside if i remember correctly).

Inside the shed there was a sink and a strange snake-ish metallic monster was slowly coming out from it.

One of the twins, the one with the "less marked" personality, decides to spend one year (during a whole night) inside the room in order to get out older than his twin.

I don't remember what happens with the monster, I think they destroyed the whole shed in the very end.

Answer

This is "Singularity" by William Sleator, written in 1985.

Plot Synopsis:

Barry and Harry Krasner are identical twins, but that's where the resemblance stops. Barry's more athletic, more aggressive - and he's the one who suggests that they house-sit their great-uncle's farm. Harry hopes that it will bring the two of them closer. And it does - because there's something chilling about the farmhouse, something that makes the locals stay far away. The twins are sure that the locked shed on the property is the reason why - but what they find inside is far more horrible than their worst nightmare.

I don't recall the monster in the sink, but I definitely remember the one twin's motivation was to become older than his brother (and a year of exercising made him buff). I believe it was 1 second = 1 hour as well because they discovered it when one of the brothers left the other inside the room for 30 seconds. The one inside the room had to eat some of the MREs that the uncle had left in there since he was in there for an entire day from his perspective.