First, let me commend you for the love and compassion you showed to your sister. No doubt it was very demanding, and you sacrificed much to take care of her. But God gave you strength, and you weren't only helping your sister - you were also serving God.
I suspect there may be at least two reasons why you feel the way you do. First, obviously you and your sister were very close - and you are now grieving her death. When we lose someone we love, our grief can almost overwhelm us - often in unexpected ways. There isn't anything wrong with this; even Jesus wept at the tomb of His friend Lazarus (see John 11:35). The Bible says that when we know Christ we won't be filled with grief "like the rest of men, who have no hope" - but we'll still grieve (1 Thessalonians 4:13).
The second reason may be because you found great fulfillment - and even joy - in taking care of your sister (although it was hard). Your duties gave you a sense of worth and satisfaction - but now you wonder if your life still has any value.
God knows your feelings, and my prayer is that you will turn to Him and discover the depth of His love for you - a love so deep that His Son willingly gave His life for you. Then ask Him to show you new ways that you can serve others for Christ's sake.