"My sister is older than my brother."
Translation:Mi hermana es mayor que mi hermano.
"Que" in this instance means "than." "De" isn't really used in this context I'm Spanish. It typically means "of" or "from." Somebody is not older from/of somebody else. Prepositions can get tricky in other languages, especially when you are going from ~150 in English to ~23 in Spanish. Knowing the right one for the job can be hard.