Terribly sorry for the delay, anonymous!
Gardevoir are incredibly protective of their trainers. Not only do they possess powerful psychokinetic abilities which they use offensively against any potential threat (as you’ve just found out), but they also possess both precognitive and empathic abilities that allow them to sense danger in the first place. These abilities have technically existed in every member of the ralts line from birth, but they grow stronger with each subsequent evolution, as do gardevoir’s devotion to their trainers.
Thus, how you proceed from here depends entirely on the underlying cause of your gardevoir’s hostility. It could be that your sister has only recently fostered murderous feelings toward you. Alternatively, she could have been planning to murder you all along, and your gardevoir has only been able to sense that after evolution. Either that, or the mere sight of your sister drives you to foster murderous feelings that your gardevoir has begun to pick up on.
If, however, there is no murder involved, it is always prudent for the trainer to explain to their gardevoir that siblings are neither threats nor objects of their jealousy (as some gardevoir do express envy towards people who receive more affection than them from their trainers). Having your sister do something nice for the gardevoir in order to establish a bond between them might help too.
Good luck, anonymous!