In “Aida’s Secrets,” a man discovers that he has been surreptitiously separated from his brother for 60 years, since they were toddlers at the Bergen-Belsen displaced person camp in Germany after World War II. And that’s just the beginning of a compelling mystery documentary that spans continents and sketchy family identities. Raised by foster parents in Israel, Izak Sagi has had a good life since the war, surrounded by people who care about him.