。
She put on her sister's jeans and looked in the mirror.
Literal
She [topic-は] older-sister [genitive-の] jeans [object-を] wore [then-て] mirror [object-を] saw.
The verb はく ('to wear, to put on') is reserved in Japanese for items worn on the lower body — pants, skirts, socks, shoes, jeans. Different verbs cover different body zones: 着る ('wear' upper-body and full-body garments), かぶる ('wear' on the head), はめる ('put on' rings, gloves), かける ('hang' glasses, scarves), する ('do/wear' for accessories like neckties). ジーンズ is a katakana loanword from English jeans. The te-form connects the wearing and the mirror-checking as a temporal sequence.