hard-working day...I think we would say a day of hard work

20-year old...You need another hyphen.

In the rules at the bottom, there is an unnecessary "who" in the last sentence.

I tried to go directly to the Indiana story from your home page, but couldn't get it. I had to go to the rules first to click on Indiana.

after a hard working day
-> after a hard day's work

a letter in her closet which said
-> a letter in her closet. The letter said that

she had immense fortune which was
-> she had an immense fortune that was

Thus, the daring youth
-> So the daring youth

His long and extremely tedious quest for the missing sibling is highlighted by all kinds of media, newspapers in particular....
Question: Is "tedious" the word you want here? It makes the game sound dull, tiresome, and uninteresting.
Question: What does it mean for the quest to be highlighted by media?

I would suggest putting the objective here, before the rules--the what before the how. The rules have no meaning until you know what the task is.

Rules:

Rule 1: Stick to the newspaper style.
-> Stick to newspaper style.

Rule 4: Each entry should contain no less than 15 lines.
-> ...no fewer than...

Rule 5: The game lasts strictly a month.
-> The game ends after one month.
or The game lasts for one month.
or The game lasts exactly one month.
or The game has a strict time limit of one month.

The who player with most postings gains the victory
-> The player with the most postings wins.
or The player who has the most postings wins.