The Mishna on our daf introduces the concept of yihud – that a man and woman who are not married to each other should not be in a secluded place together, for fear of sexual impropriety between them. According to the Tanna Kamma, a man should not allow himself to be alone with a woman, or even with two women; a woman cannot be alone with a single man, but if there are a number of men, then there is no problem of yihud. According to the Gemara, we fear that the two women will allow sexual relations to take place between the other man and woman, but that men would be embarrassed about engaging in relations in front of one another.
Our Gemara cites Rav, in a quote brought by Rav Yehuda, who limits the leniency with regard to a woman being alone with a group of men to situations where the men are known to be kesheirim – “kosher,” i.e. reliable people. To support this contention Rav Yosef points out that that it is not unusual for a group of men to conspire to steal something together, with no embarrassment from one another.
In searching for a source for the halakha of yihud, the Gemara brings Rabbi Yohanan quoting Rabbi Yishmael as saying that a hint (remez) for this law can be found in a passage in Sefer Devarim (13:7) that is discussing idol worship. The pasuk warns that a person might be enticed to idol worship by his brother, the son of his mother. Why does the Torah suggest that the enticement will come from a brother from his mother rather than from his father? The implication is that brothers from the same mother can spend time together – and with her (yihud does not apply to parents and their children) – but brothers who share a father but have different mothers could not be together, since there would be a problem of yihud.
The Talmud Yerushalmi brings the same passage as the source for yihud, but emphasizes the entire pasuk, which includes not only a brother but also a son or a daughter who entices you ba-seter – in secret – to worship idols. From here we can conclude that immediate family relatives can be together ba-seter.