“as if [she were] a sister or as if [he were] a brother”. My guess is that whoever the phrase is referring to has the same legal status as a sibling. It could also be translated “such as a sister or such as a brother,” more in the sense of “for example,” though other Latin phrases such as “e.g.” are used more in that context. ETA: Half Man Half Wit’s translation is also correct, word for word, but again I’d expect similis or something like it if that’s the meaning that was intended.