I'm just now realizing the importance of articles.
As I've been learning Latin, I've realized that articles are very important in carrying a message. For example, "Illi familiares benigni sunt," could mean "They are kind relatives," which implies that they are not the only ones, or it could mean "They are THE kind relatives," which implies that every other relative is a downright terrible person that treats you like actual garbage.
Edit: I intentionally gave the two most extreme examples I could think of so as to properly convey the magnitude of difference that articles can make.
Yes, and languages usually EITHER have articles OR multiple noun endings for different cases to help convey the sense of what you want to say. Very few have both.
Latin can use demonstrative pronouns (this, that, these, those) when it's important that it's not ambiguous and might otherwise lead to confusion, but usually context is enough.
It could also mean "Those relatives are kind".
Language relies on the listener to interpret potentially ambiguous sentences in the way the speaker intended by using a shared context, in order to allow compact and efficient communication -- when this fails, the speaker has not established the right context, or not provided enough explicit information in the sentence.
I think that if the sentence was taken to mean "they are the kind relatives", it would only be in the sense of having just spoken about them, so you are referencing a previous sentence (essentially, "they are the kind relatives, of which I spoke previously"). I don't think it can mean "they are the only relatives who are kind". I don't know enough Latin to say how you would say that, though. My guess would be adding a "soli" (Illi familiares soli benigni sunt) to say that they alone are the kind relatives. Or perhaps adding a "qui" (Illi familiares qui benigni sunt) to say they are the relatives who are kind (implying other relatives are not).
These are just guesses at how you might say it - hopefully someone who knows more will come along and explain.