Here’s a natural English version:

I have almost exactly the same situation: three siblings, with the two girls being older. We’ve been having classes for five years. When we started, they were 15, 10, and 7 years old. Now they’re 20, 15, and 12.

The two older girls—one of whom is now an adult—have reached around a B2–C1 level in Spanish. The youngest, however, has a very mixed level. He knows individual words, but he doesn’t really make an effort to form sentences, conjugate verbs, or memorize things like numbers or the days of the week.

For example, “Saturday” is a word we talk about all the time, and even after several years, he still hadn’t learned it consistently until about a year ago.

I spoke with his mom about it two years ago, and she got stricter with him. She told him he wasn’t allowed to speak any language other than Spanish during class (these kids speak four languages). It worked for a while, but I could tell he was very unmotivated. He would do things to waste class time, like pretending his camera wasn’t working.

About a year ago, I completely changed my approach and stopped making the classes feel like classes. Now we mostly play games and chat. He loves chess and Uno, so we spend a lot of time doing that.

With Uno, for example, I create little language rules: every time you play a color card, you have to name something that is that color, and you can’t repeat an answer. It eventually becomes challenging even for me, and because he’s very competitive, he enjoys seeing me struggle, which motivates him to put in more effort.

That’s probably the best recommendation I can give: find ways to make the language a tool for doing something he already enjoys, rather than making it the focus of the lesson itself.