Am I dumb? Are Sasha and Alexander the same person?

i think so, maybe it was a childhood nickname?

It seems like it, at some point I thought it was his brother since in the family portrait there is another guy.

sasha is shortened version of name Alexander and alexandra in russian

Yes it’s the same name. So in Russia, (and probably most Slavic countries) Alexander is for some reason always shortened to Sasha. It’s even become a meme in some reels . In English though it would technically be Alex but ya

actually in Poland you would most probably shorten Alexander to Olek, though some people in gen z decide to go with the English ver and call themselves Aleks