I would be a bit surprised by them since Amir is very obviously Arabic while Alexander is primarily Germanic? There’s nothing wrong with that but it’s definitely something that’d catch my attention because it’s a pretty unexpected pair of sibling names!
I’m not sure what names to suggest since these names are so different! For a brother to Alexander, I’d be inclined to suggest another familiar name like Max / Maxwell, Charlie, Oliver, Liam, Nicholas, Sebastian, Elliott, Julian, Roman, Theodore, etc. while for a brother to Amir I’d be thinking about Arabic names that work well internationally like Iskandar, Idris, Haris, Kamil, Laith, Isa, Zayn, etc.