这张地图很难阅读。
你有城市的地图吗？
我不相信他的故事。
    ->我不相信他说的( I don't believe what he said/
我最喜欢的季节是夏天。
花园里有很多美丽的花。
他和他的哥哥没有很好的关系。
    ->他和他哥哥关系不太好

Are these sentences correct?

1.这张地图很难阅读。

这张地图很难看懂。

2.你有城市的地图吗？

你有这座城市/该市的地图吗？

3.我不相信他的故事。

4.我最喜欢的季节是夏天。

5.花园有很多美丽的花。
花园里有很多美丽的花。

6.他和他的哥哥没有很好的关系。

他和他哥哥关系不怎么好。

Note: We can aslo say 他的哥哥 as 他哥哥, without the character 的.

All of your sentences are nice and understandable, I just tried to make them sound more native...

1. This map is very difficult to read.
2. Do you have a map of the city?
3. I don't believe his story.
4. My favourite（最喜欢的） season is summer.
5. The garden has a lot of beautiful flowers.
6. He doesn't have a good relationship with his older brother.

Are these sentences correct?

→為口語用法

1. This map is very difficult to read.
2. Do you have a map of the city?
3. I don't believe his story.
4. My favourite season is summer.
5. The garden has a lot of beautiful flowers.
6. He doesn't have a good relationship with his older brother.