Sie.

From Proto-Indo-European language pronominal stem -i and initial s. In the early Germanic languages, the sie is formed according to the above pattern, compare Gothic 𐍃𐌹 (si). The si/se was originally used to strengthen demonstrative pronouns, which is still preserved today in dieser, diese, dieses. The sie probably found its way into German primarily via the Old Saxon se, which developed as a plural form of the simple demonstrative thia. In Old High German, the forms si and sie are then found, which are also retained in Middle High German. In Modern High German, the many alternative forms (e.g. Middle Low German se) disappeared in favor of the modern sie. Due to the grammatical equality of sie and Sie, the capitalization of the pronoun came into use in the 17th century to differentiate between them when used as a form of address.