Sistar

Very good female friend that you have known for years. In the context of the COVID-19 and the regulations against the gathering of people from different families, it means that the person in question (the sistar) can be considered in the same family as you are. Thus, it bypasses the COVID-19 regulations and enables face-to-face meetings between all the brothar and sistar that know each other.

Police : Gathering of people from different families is presently not allowed during COVID-19, please distance yourselves
Me : I understand, but she isn't anyone, she is my sistar which allows us to meet-up despite the current crisis
Police : I see, my mistake, please proceed

Brothar

Very good male friend that you have known for years. In the context of the COVID-19 and the regulations against the gathering of people from different families, it means that the person in question (the brothar) can be considered in the same family as you are. Thus, it bypasses the COVID-19 regulations and enables face-to-face meetings between all the brothar and sistar that know each other.

Police : Gathering of people from different families is presently not allowed during COVID-19, please distance yourselves
Me : I understand, but he isn't anyone, he is my brothar which allows us to meet-up despite the current crisis
Police : I see, my mistake, please proceed