YOU SAID:

Big sister is whatching the muffin

INTO JAPANESE

姉はマフィンをむしゃむしゃ

BACK INTO ENGLISH

My sister is munching on muffins

INTO JAPANESE

妹がマフィンをむしゃむしゃ食べている

BACK INTO ENGLISH

My sister is eating muffins

INTO JAPANESE

私の妹はマフィンを食べています

BACK INTO ENGLISH

My sister is eating muffins