@middlechild today will no doubt be difficult for you. Try & use it to remember the good times you had.
It’s great that you have some good friends that are there for you. Keep talking to them it will help. I’d be lost without my friends.
Your sister is probably dealing with things the best way she knows how. When my dad passed away my sister went back to work next day which i thought very strange but she said she needed the distraction. All you can do is be there for her when she needs you.
I know it’s a cliche but time is a healer & you will get there. Perhaps have a talk with your GP if you’re really struggling.
Sending you my very best wishes.
Ann xx