Initially Celie writes her letters to God, but after she loses faith in her religion, she pens the majority of her letters to her sister Nettie.
She and Nettie are separated when Celie is forced to marry Mr.______, a man that used to be Nettie's boyfriend. After he and Nettie argue, she is forced to take shelter with Samuel and Corrine, missionaries who take Nettie with them to Africa.
Celie is unsure about whether or not Nettie is recieving the letters, but continues to write them anyway.
That answeris not correct.
Celie DOES write her letters to God initially, but when she discovers the letters that Mr. ____ (or Albert) have been hiding from her (from Nettie), she starts to write to Nettie, not knowing if Nettie receives them.
Nettie and Albert were not dating at any time. Albert always wanted to marry Nettie but she was too young, and Albert was forced to pick Celie. After Celie moves out, Nettie moves in with Celie and Albert. When she realized she couldn't stay with her sister and Albert, she joins Corrine and Samuel (she is not forced) on a missionary trip to Africa.
She writes to God because she is lonely and has no one to talk to. She writes to Nettie after God because she has hope that Nettie is alive and well.