。
I've got to wrap up Sis's dinner in plastic wrap before I forget.
Literal
I [topic-は] sister [gen-の] evening-meal [obj-を] wrap [into-に] wrap-[completely-てしまう] [if-not-ないと].
ラップ is the loanword for 'plastic wrap / cling film,' derived from English. 包んでしまう combines 包む ('to wrap') with the auxiliary ~てしまう ('do completely / end up doing'), here emphasizing 'get it completely wrapped up.' The final ~ないと is the truncated 'have to' conditional — the implied consequence ('otherwise it'll go bad / I'll forget') is left hanging. お姉ちゃん is the casual/affectionate form of 'older sister.' A natural everyday monologue.