i am so sorry to hear about your loss. it sounds so traumatic what you have been through. your sister was so young and no doubt you cared a lot about her. i want to tell you that it is often normal to have flashbacks and experience rough sleep after a traumatic loss.
my mum was in her 40s when she passed. her death was sudden and unexpected. its been many years now but every so often i have had to seek the help of a psychologist so i could work through the associated trauma. perhaps this is something that you can also consider and will find useful.
when i felt like crying, id just let the tears flow. there was a lot of anger and guilt inside of me which i had to put to constructive use. i took up journalling and yoga. journalling helped me to let my feelings out on paper and make sense of what was going on. i journalled every night and it helped me sleep without thoughts lingering in my mind. yoga helped me to physically shift the emotions.
i think youre so brave and i wish you strength as you work through your grief, loss, and associated trauma. i hope this helps.