Complex sentence: Nettie also learns that Alphonso is actually only Nettie and Celie's stepfather, not their biological father, who was a storeowner whom white men lynched because they resented his success.
Split into simpler sentences: Nettie also learns that Alphonso is really only Nettie and Celie's stepfather, not their real father. Their real father was a storeowner whom white men lynched because they resented his success.