Confused with the interpretation of the restrictive relative clause

The sentence:

My sister who is from Chicago visited me last weekend.

The interpretation from YouTube: I have more than one sisters and I am specifically talking about my sister who is from Chicago.

Question: If I say,

"The car that I bought last week is red"

Will people think that I have more than one cars? (Ooops, do I have to add s at the end of car in this sentence?

I am a nonnative speaker who enjoy learning English. Please help me with this interpretation.

There are two types of English relative clause. Their traditional names are defining (or restrictive) and non-defining (or non-restrictive). ‘The Cambridge Grammar of the English Language’ uses the terms integrated and supplementary, which seem to me to explain their difference more clearly. Integrated relative clauses are essential to the meaning of a sentence and cannot be omitted if the meaning is to be retained. Supplementary relative clauses provide additional information. When a supplementary relative clause is removed, a sentence with the same underlying meaning remains.

It is the convention to set off supplementary relative clauses with commas, so that if your first example had included a supplementary relative clause it would have appeared as ‘My sister, who is from Chicago, visited me last weekend.’ That would have left little doubt that you had only one sister. Without commas, the clause ‘who is from Chicago’ becomes an integrated relative clause and leaves the reader with the strong impression that you have more than one sister but that the one who visited you last week-end is from Chicago.

The sentence ‘The car that I bought last week is red’ contains the integrated clause ‘that I bought last week’. It tells us that of all the possible cars you could be talking about you are talking about only one, that is, the one you bought last week. The sentence about your aunt who is from Chicago doesn’t deny the possibility that there might be other aunts elsewhere. Similarly, the sentence about the car you bought last week doesn’t deny the possibility that there might be other cars elsewhere. You aren’t contrasting the car you bought with any other cars you may already own, but with all the other cars in the world. If we were to treat the clause as supplementary and write ‘The car, that I bought last week, is red’ we could remove the clause and be left with ‘The car is red’ and that would be a grammatical sentence. But it conveys different information, lacking as it does the crucial point about which particular car you’re talking about, that is, the one you bought last week.

The two sentences you presented aren't really related in any way.

The sentence "my sister who is from Chicago" doesn't necessarily mean that you have more than one sister, but since you define which sister exactly (the one from Chicago), it is expected that you have more of them, otherwise you wouldn't define which exactly.

To not define which exactly, yet still use the part, you'd surround it with commas, to make that part "secondary", like this:

My sister, who is from Chicago, the rest of the sentence.

As for the sentence with a car, it doesn't say anything about how many cars you already have. I think the explanation I provided above will help you resolve everything you need to know. If not, comment and I'll expand my answer to provide further information you may need.

For an interpretation of the sentence with the car, if it is parallel to the first sentence, you could read it as the car is red, with an added specifier of that I bought, which could imply that I own many cars, but only one that I bought. Typically though, the car that I bought implies only that there are many cars, but only one that I bought.

I'd say the YouTube interpretation is wrong. When you say "My sister who is from Chicago ... ", this does NOT imply that you have more than one sister.