In Igros Moshe he mentions the Shailo of a brother and sister being together in yichud-he said they can be in the yichud for a certain amount of time (i think it’s between 2-3 weeks).
He mentions other yichud cases like, and is lenient in many of them as there could be a situation where a second male can come back any time (unannounced) and it will no longer be yichud (mentioned in Talmud Kiddushin).