Perhaps I am mistaken, but I I recall back when I studied hilchos yichud that one man MAY be misyached with two women as long as one of them is someone with whom he could be misyached if she were alone. In this case, a brother can, of course, sleep in the same house as his sister, and therefore can also sleep in the same house as his sister and his sister’s friend, provided of course that no other yichud issues develop – i.e., sister leaves friend alone in the house with brother while she goes out for a specified extended time; or brother and sister’s friend seclude themselves in the same room – locked from the inside – while sister is still in the house.
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