If a brother comes for a 3 week visit to his married sister, would there be a problem of yichud if the husband is not in the city during the day and returns late at night? There are 5 children between the ages of 1 and 10 in the house but they would be sleeping by early evening.
No, in these circumstances there is no problem of yichud between a brother and a sister.
See Iggros Moshe, Even Ha-Ezer Vol. 4, no. 65 (11).