If you don't have enough faith or belief ... take some time out, alone, so you can be with the deceased spirtually, and pretend ... that there IS life after death, and that there is a spiritual world, and that ALL is possible in the spiritual world.
Then ... talk to your loved one, and tell them what you need to tell them.
Fortunately, I have a very liberal spirtuality, and believe that all things are possibly with that part of life. that's just my willingness.
I'm so sorry about the loss of your sister. I lost my sister a year ago and we were best friends in the world. I would call her every day. We thought we were so cool. There are times when I forget and want to call her and then I remember. I know how you feel. Please take care.
This is just my spirituality. I often believe in God, and I sometimes think that if there is a God, he (or she) can do all things ... including allowing our deceased family members to HEAR us. My faith fluctuates. However, this line of reasoning leads me to feel good about praying FOR and TO my sister. I have a very open mind when it comes to spirituality. While I consider myself pretty grounded, I still sometimes say things to myself like "if this world is possible, why would not other worlds or realms exist?" AND "if there is a God, and he or she is so powerful, then perhaps ALL THINGS are possible with God". Perhaps this could mean that our loved ones are safe, and perhaps chuckling at our sadness or fear. At the very least, I know that my sister is no longer in pain, and is at peace. I hope that I can someday have as much courage and faith as my sister.
Something else that I would like to add is a suggestion for grief healing, especially if a survivor cannot or does not want to seek outside help. I have a hard time just relaxing, so what I did was buy some guided meditation CD's, with music, specifically to deal with getting consolation and support, so that I could cry, like many people can do more naturally. I do not want to recommend any particular provider, but I would just suggest going to a place like Amazon and doing a search for CD's and or books, using terms like HEAL GRIEF. There is a lady who is famous for her work, and you will probably find her right at the top of the results. very soothing and therapeutic, with continued use.
Sorry to hear about your loss. I know its painful. I am a male. My youngest sister died on the last day of 2006, at 43, due to lung cancer. (She was a big smoker.) I had and still have feelings of loss and sadness.
For me, being stuck in grief is the worst; just crying is such a welcome release. If you are similar, I hope you can release your grief. But, don't feel like you need to rush. If I can suggest something, please do not isolate. Posting in forums like this is OK, but consider activities like grief support groups; or seek out anyone who can support you in just connecting and sharing your loss. There are counselors who specialize in this. I did a number of things, including doing a 10 week grief group at a local church, even tho I was not that active there at the time. just google on grief support group and your city.
I hope its OK to add, that if your sister did end her own life, I am sorry that she was in that emotional state. It happens much more often than what we hear about publicly. Some people are in pain and see no other way out. In any case, just know that you are not alone. I send my support. FM