。
I learned cooking from my older sister.
Literal
Cooking [topic-は] older-sister [object-を] teacher [into-にして] learned.
The pattern ~を~にして ('making X into Y') casts the older sister into the role of teacher. It's a を+にする construction where にして is the て-form linking to the main verb 習う. The subject (私) is omitted — the polite ~ました ending implies the speaker is talking about their own experience.