From a very young age, Elsa and Anna were extremely close; they shared the same bedroom and often played with each other, using Elsa's ice magic to enhance the fun. However, this all changed when Elsa accidentally struck Anna with her powers. Though Anna made a full recovery, Elsa was traumatized by the incident and agreed to her father's decision to keep her hidden from the world and separated from Anna, seeing it as necessary to keep everyone safe. As the years went on, Elsa repeatedly turned down Anna's constant attempts to spend time with her, and a rift grew between the two sisters.

On the day of her coronation, Elsa found herself in Anna's presence and both sisters were initially only capable of stealing awkward glances. Elsa eventually seized initiative and engaged Anna with friendly banter; the two appeared to make real progress towards ending their estrangement. However, when Anna revealed that she intended to marry Hans, Elsa was shocked by such a sudden proposal and refused to bless the marriage. This prompted her to order the castle gates closed, declaring the coronation party to be over. Distraught over the prospect of being isolated once more, Anna challenged her sister, questioning Elsa's need to maintain distance. Losing patience, Elsa inadvertently revealed her powers, nearly striking Anna and some of the guests with a wall of ice spikes. Having spent over a decade trying to contain her powers so as to not harm Anna, Elsa was horrified that her efforts had nearly been in vain, and despite her sister's protests, she fled Arendelle, unwittingly plunging the kingdom into winter in the process.

While Elsa saw her flight from Arendelle as necessary, she still missed Anna greatly, even reconstructing the snowman they had built as children. As such, when Anna managed to locate her at the North Mountain, Elsa was initially happy to see her sister. But as the memories of the past slowly began to overwhelm her, Elsa insisted that Anna leave to remain safe. Anna tried to convince Elsa to return home by reminding her of their past friendship and her duty to Arendelle. However, Elsa's increasing fear resulted in her striking out with her magic, freezing Anna's heart. Unaware of the severity of Anna's injury, Elsa reacted aggressively to her sister's refusal to leave by creating a giant snow monster, Marshmallow, to physically remove her.
Anna thaws out

With Anna's sacrifice, Elsa realized that love was the key to controlling her abilities.

Though Elsa never intended to, her attempts to keep Anna safe from her powers only resulted in her sister being physically and emotionally hurt. Elsa eventually discovered that she had froze Anna's heart, causing her to slowly transform into solid ice. However, in her last moments before completely succumbing to Elsa's curse, Anna threw herself between her sister and Hans, who was intent on killing Elsa to usurp the throne. Devastated upon seeing how her magic had afflicted Anna, Elsa embraced her sister's frozen body while sobbing.

When Anna's frozen form suddenly thawed out, Elsa was at once surprised and relieved to see her sister alive and well. Touched by Anna's sacrifice, Elsa realized that love was capable of thawing and determined that it was the key to controlling her abilities. After Elsa used this knowledge to finally restore summer to Arendelle, the two sisters embraced, feeling closer than ever. Later, in the castle courtyard, Elsa and Anna skated together, as they did during their childhoods.

After years of being apart, Elsa wanted to make up for lost time by giving Anna an unforgettable birthday, despite being sick at the time. Though she refused to allow the cold to hinder her plans, Elsa eventually succumbed to its effects and felt that she had ruined Anna's birthday. However, she was grateful when Anna said that the best birthday present was the chance to take care of her older sister.

Credit: Frozen Wikia