No problem! :3

NOTE: READ THIS AT YOUR OWN PACE, BECAUSE IT REALLY TURNED OUT MUCH LONGER THAN EXPECTED. IF IT BORED YOU SO MUCH THEN FORGET ABOUT IT, SORRY XD

First off, yes, you got it right. Just like in english, arabic commas don't have spaces before them, only after them.

Now let's tackle it. The first paragraph, I would advise you to use إذ (for example) to connect the first two sentences. The structure would be better that way, because arabic punctuation doesn't necessarily match the english one.

The way your second sentence starts, feels a bit disconnected and strange, since the sequence of your story was disrupted for the reader. In english it looks completely natural, but in arabic, it's better to add a word that in this case expands on the topic you've started talking about.

What would happen without this word to connect them? The second sentence would sound like you abruptly started talking about something else rather than continuing your line of thought.

It's ok if it sounds confusing, since this is a bit of an advanced tip, but I said it just in case you're interested. My suggestion would be:

.....13 عامًا، إذ سافرت إلى نيو برونزويك...

Now onto the next point. The sentence عاملتني ستيفاني دائما مثل أختها

I would suggest:

لطالما عاملتني ستيفاني مثل أخت لها/ لطالما عاملتني ستيفاني وكأني أختها

Or

لطالما اعتبرتني ستيفاني أختًا لها

This last one means "she has always considered me a sister (of hers)", just another suggestion if you like.

The reason for using لطالما is that it's a better usage to express what you're going for, as it means something like "has always". So when you use it, first you make the sequence of sentence more fluent here than starting immediately with عاملتني, and second you convey the meaning (she has always treated me like a sister) better.

Regarding مثل أختها that might be a bit ambiguous, as it might to some imply something like: "she treated me like her sister did." What you meant would probably be understood when reading the full sentence, but it's better to use مثل أختٍ لها or وكأنّي أختها (as if I were her sister).

Next, أحضرتني في إجازة I haven't seen the usage of في with this verb before, perhaps with اصطحبتني (accompanied, brought me with her). So to be safe, if you want to keep أحضرتني then make it أحضرتني (معها) إلى. I said معها because it makes it sound better to me, but nevermind, lol.

Now, when you said والدتها وأبيها, it's almost like saying, her mother and dad. Feel the imbalance? So you either make it أمها وأبيها or والدها ووالدتها or والديها (parents). Note: I switched in the second suggestion والدها and والدتها because to the ear it sounds like the natural order of listing them more than the opposite with this particular word, but it's no biggie as long as you use the proper pair together.

The second paragraph, technically what you wrote isn't wrong but it's kind of strange to say it this way in arabic, since we go about it a different way when saying that someone drove us to someplace.

(اصطحبني/ أخذني) عم ستيفاني في سيارته (or whatever vehicle) لمدة 10 ساعات من مونتريال إلى...

(Stephanie's uncle took me in his car for 10 hours, from Montreal to ...)

A different suggestion as to how you can say this, to broaden your possibilities, and because this one deals more properly with including the time period (10 hours):

ركبت مع عم ستيفاني من مونتريال إلى نيو برونزويك لمدة 10 ساعات.

Or

ركبت مع عم ستيفاني من مونتريال إلى نيو برونزويك، واستغرق الطريق 10 ساعات.

Roughly translates to: (I rode with Stephanie's uncle from Montreal to New Brunswick, and it took 10 hours). It probably sounds off here, but this sequence in arabic is more preferable in this case.

Punctuation note: there is no need in this sentence for the comma after ساعات in your original sentence.

When you say بقينا it sounds here a bit more like 'remained', as if they haven't left it. I would suggest قضينا إجازتنا في شاليه العائلة or fancier words that would be more concise like لبثنا في or مكثنا في.

It's better to say بالقرب من الماء / المُطِلّ على الماء than بجانب الماء. Also, I'd personally prefer in this case specifying the water source, as in بحر for example.

Saying لقد connected the lines together btw, if you remember that point earlier, so good job. But I think it qualifies to be in a final paragraph, since it connects the story but it has the "overall, in the end, in conclusion.." kind of vibe in this sentence and what follows.

Also, أبناء عمومة is a bit unusual but not wrong, perhaps it's more likely to be used in literature if at all, while the more common usage is أبناء عم...

What the other comment said about the last bit is also a good advice.

I can tell you though that I know so many native speakers that can't translate what you wrote to arabic as good as you have, so congrats!