Translation:My elder sister stopped her diet after a week.
Just my to cents here, but I used Google Translate (I try not to depend on that) and vdict.com.
Apparently, "chế độ" = "regime" or "regimen". Which kinda makes sense when it refers to "diet". Do we need "chế độ" with "ăn kiêng"? I have no clue. I thought maybe "ăn kiêng" by itself might also refer to the eating disorder, anorexia. However, my two (rather weak) sources gave anorexia another name.
Anyone have a credible Vietnamese dictionary to confirm my findings?