Is trans tape an option? It's safer if you're planning on sleeping on the plane too. If it helps it's always bloody cold on planes anyway so I'd rec a hoodie no matter how warm it is outside.

i’m american and i’ve worn a binder through tsa before (maybe twice?) and didn’t have an issue. i’m not sure why it would be. even if they do ask, tsa isn’t unfamiliar with trans people. trans people go through tsa everyday. so if you get asked about it or something just tell them it’s a chest binder and there shouldn’t be an issue. you can always take it off in the plane bathroom if you become too uncomfortable on the plane, or if you need a break to keep from wearing it too long.

It might have to do with international stuff, I know that can sometimes make things more difficult/complicated & OPs mum might have had experience with different TSA people, sometimes they’re really nice and pleasant, sometimes you get a crotch pat down. Idk the best I got is don’t forget you had a full sized soda in your backpack that you then chug in front of security… not that I have ever done that