If you want a TL;DR explanation to give your cousin on why she's not that great, tell him to think of it like this. She's a glass cannon. Good attacks like LK if you sweetspot (which have been severely nerfed over the last few updates), but her movement and other aspects of play are super punishable - plenty of time for opponents to react, and high-ish commitment in many scenarios. Some aspects of her playstyle may seem unintuitive, but so do a good 80% of the non-Melee top tiers, if you've never seen a good player using them before. Specifically on Bair and B-throw though...no. They're not OP. There are stronger kill throws out there, and there are other moves like knee of justice that have more guaranteed kill setups on other characters.
Zelda stuff just happens a bit more slowly than with other characters, so there's more emphasis on player errors and people don't like accepting that they made a mistake, or they feel like their quicker play (more bad things in a short period) is more skillful and should justify beating slower good play lol. I say this not as a Zelda main dealing with salty opponents, but as someone who understands Zelda as a character, and can't seem to beat another local Zelda in the ditto or with my secondaries at all, even though I beat players that beat him lol. I'm salty too, but at least ik I'm just getting outplayed, even if interactions feel stupid like the GW mu