I'm not sure if ithat's somethinga good or bad thing for me.

WheIn September comes, all of, my friends come backhome from the beach or from other placeir summer vacations and that makes me happy.

there's nothing wrong with saying "from the beach or from other places" but in casual conversation, from summer vacations would be enough.

In summer, I have nothing to do, because my girlfriend and my friends are always out from Seville.of town.

if you want to mention Seville, you could say something like "usually leave Seville"

So these areis is good news.

While your intuition is right knowing that "these" refers to multiples of a thing and you listed multiples of things above, grammatically and in casual conversation it makes more sense to use the singular "this".
Functionally, this sentence is saying something like "this news (that both my friends and girlfriend are coming home from their summer vacations, and any number of things) is good news"

However, the day my brother iswill be leaving Seville is closer to ussoon.

He's going to live in Praga for onea year.

I've just realised that his departure is not a jokHis departure is becoming more real to me and it's closer than I thought.

this is the right way to use "not a joke" in a sentence, but "not a joke" conveys a different type of seriousness than this context
Example 1 "This conversation isn't a joke, I need you to take this seriously." Example 2: "Skydiving school is not a joke. It's way more difficult than I was expecting."
"becoming more real", on the other hand, conveys seriousness, but in a less potentially angry way and in a more contemplative, thoughtful, or sad, longing kind of way.

Since we klearnewd he was leaving in the ninth ofon September 9th, we've seen this date aso distant until now.

Due toBecause of this, I've cried a lot today.

Due to this is more matter-of-fact and formal. because of this is a bit more casual and personal, which is more appropriate for this type of topic.

Plus, he isn't looking forward to going because he's going to miss many people here, like his girlfriend, his family, his friends....

However, heit's not able to say no totoo late to turn down thise experience now, it's too late.

September has arrived to(in our lives)

(1) The preposition to use here is "in".
(2) You may omit "in our lives" to be more concise, as it might be somewhat redundant.

In summer, I have nothing to do, my girlfriend and my friends are always out ofrom Seville.

The more natural phrase is "out of (a place)".

So these areis is good news.

It being September is only one piece of news, so the singular is expected.

He's going to live in Pragaue for one year.

Did you mean Czech capital Prague?

Since we knew he was leaving ion the ninth of September, we've seen this datethis date had seemed so distant until now.

(1) For specific dates, we use the preposition "on".
(2) I've rephrased the last part of your sentence so that it sounds more natural.

Plus, he isn't looking forward to going because he's going to miss many people here, like his girlfriend, his family, his friends....

Instead of "like", you can alternatively also use "such as".

However, he's not able to say no to this experience now, it's too late.

I'm not sure "experience" is the most appropriate word to use here. In fact, you can simply remove it. The reader will be able to infer what "this" refers to.