Leenia - where did you get the definition "sister"? Deartháir means brother. Deartháirín means "little brother" or "dear brother". Yes it's true that the diminutive "ín" can be added to names to give a feminine version (Pádraig for a boy, Pádraigín for a girl/ Gearóid for a boy, Gearóidín for a girl) but that isn't the case here.
sound file for "dearthair" https://www.teanglann.ie/ga/fuaim/dearth%c3%a1ir   (the "ín" adds an "een" sound)

I don't know how a translation site came up with "detected". Bleachtaireacht is the word I know of for detection. And a detective is a "bleachtaire"