One thought process is that something is missing in your waking life- a quality or aspect you would like to have but currently don't.
It's possible that your sister possesses a quality you yearn for. What is it that you see in her as being so special?
The death of a sibling could mean that you lead a busy life and have not dedicated any quality time to them lately. It might help your relationship to let them know what they mean to you and spend some time remembering the good times you’ve had together. These shared memories may help you to bond better in the future.
Another interpretation is that you feel a disconnect from your sister. Perhaps it's important to spend more time with her because she needs your support right now or to build any bridges if the reason for your distance is due to a fall out.
Have you have severed all contact with your sister lately? Maybe she no longer has a part to play in your life and don't want to see her again. If you are dreaming about her- it could mean that you regret your decision to part ways or are adjusting to life without her.
It might have nothing to do with your sister but symbolic of something that has ended abruptly in your life- a job, a friendship or a relationship for instance.
It's also possible that your life has evolved or changed lately rather than a part of it ending. It may be a more organic process.
Do you feel there is something wrong with your relationship with your sister at present? If so, perhaps you need to check in with her and express your concerns. She might be feeling the same way. If you miss her or worry about your relationship falling apart- it might be wise to nip this in the bud before it gets any worse.