This video talks about the issues with fridging quite nicely. Basically the problem with fridging, fundamentally, is that it takes a character and then uses their death purely as a plot device for another character. It's killing off a significanr character casually, with the main emotional concern being how the death impacts other people and not the fact the character is gone, and usually that impact is brief and doesn't last. She used the example of Lukes aunt and uncle in a new hope. The people who raised him were brutally murdered and his childhood home burned down and there's no indication he ever thinks about it again a few scenes later. She identifies that as a text book example of fridging, and that fridging ultimately is a sign that the writer didn't really think the character was as important as whichever character their death was set to motivate, and that for various cultural bias reasons, this tends to happen a lot more to women, even though the crux of the trope can apply to men as well.
Given that, I still argue that the death of Mayrina's brothers isn't fridging because they're barely even characters. We see them for one scene and they're gone. And one of them is kind of a jerk to us, so the chances of us forming an emotional connection at all are low. Their characters exists solely to make us feel sad. And I don't thinknthey do a good job of that
