I'm little confused about these tags, because while sisters and brothers are described like this:
Images or animations depicting at least two male characters who are siblings.
then brother_and_sister has slightly different meaning.
Exactly what it says on the box; this tag indicates an image containing a male and female sibling pair. The specific presence of this tag generally indicates an incestuous image, in which the brother and sister are having sex.
I was about to change it to definition, similar to 2 others, but then I checked, that all definition were given by the same person. Is there any reason why brother_and_sister is treated differently than sisters and brothers?
I would personally opt for treating it exactly the same. If someone want incest, xe could search for brother_and_sister incest
EDIT:
Also I think that there should be implication.
brothers -> male
sisters -> female
brother_and_sister -> male female
Updated by Rainbow Dash