Sadly that's another thing that just doesn't translate well to English. Big Sister Matoya is really, REALLY awkward in English and not something most native English speakers would say to someone in that position. At least not if they were older than five years old. There's really no natural equivalent in English for it that doesn't come off as forced.
Mind 'Master' isn't a good equivalent either as while it denotes respect it's also cold. I'm not quite sure how they could have translated that better honestly, except maybe 'Miss Matoya'.