How to deal with broken heart syndrome?

So sorry for your loss.

This site has a good article if you haven't seen it yet. You may want to talk to your doctor about medication.
https://www.mayoclinic.org/diseases-conditions/broken-heart-syndrome/symptoms-causes/syc-20354617

It is legitimate, and you are heart-broken, literally aching due to the sudden loss, maybe even thinking that it was so unfair and harsh....for her and for you and others who feel similarly. There is little-to-no comfort in knowing that your sister is outside of the realm of life's problems and safe from all harms.
As the article says, and it takes energy and force that you probably feel you don't have and can't wring out of yourself, being with someone who can listen to you, even if you both remain silent, might help. If you can unburden yourself with words, that often helps a lot. Going for walks, hikes, taking photos of beautiful things, writing, maybe in a journal, ...these things can help. Look at a nice portrait of your sister and talk to her...she's present as much as you. Tell her what you feel. She's a most patient listener.