You should be able to at least toast your own bread.
You're a big girl. Why can't you do it by yourself?
If you're going to complain, do it yourself!
この場合、次のような言い方ができますよ。
ーYou should be able to at least toast your own bread.
「自分のパンくらい焼けるようになってよ」
ーYou're a big girl. Why can't you do it by yourself?
「お姉ちゃんなんだから、なんで自分でできないの?」
ーIf you're going to complain, do it yourself!
「文句言うなら自分でやって!」
ご参考まで!