In Genesis 25 we meet Isaac who marries Rebecca. Rebecca conceives and gives birth to twins Esau and Jacob. Isaac is above sixty years of age when the chapter closes.
Genesis 25 NASB
20 and Isaac was forty years old when he took Rebekah, the daughter of Bethuel the [o]Aramean of Paddan-aram, the sister of Laban the [p]Aramean, to be his wife.
In Genesis 26 Isaac goes to Gerar during a famine in Canaan. He conceals his marriage to Rebecca and lives there for some time. It seems here the twins had not yet been conceived for there is no mention of Rebecca having any children.
Genesis 26 NASB
6 Now there was a famine in the land, besides the previous famine that had occurred in the days of Abraham. So Isaac went to Gerar, to Abimelech king of the Philistines.6 So Isaac [h]lived in Gerar. 7 When the men of the place asked about his wife, he said, “She is my sister,” for he was afraid to say, “my wife,” thinking, “[i]the men of the place might kill me on account of Rebekah, for she is beautiful.
In Genesis 27 we now meet Isaac an old man who was almost blind. Jacob beguiles him and receives the blessings
Genesis 27 NASB
27 Now it came about, when Isaac was old and his eyes were too dim to see, that he called his older son Esau and said to him, “My son.” And he said to him, “Here I am.”18 Then he came to his father and said, “My father.” And he said, “Here I am. Who are you, my son?” 19 Jacob said to his father, “I am Esau your firstborn; I have done as you told me. Get up, please, sit and eat of my game, that [k]you may bless me.
The Gerar incident should have come before the birth of the twins.
Genesis 25 Isaac is above sixty years
Genesis 26 Isaac is between forty and sixty years
Genesis 27 Isaac is about one hundred and sixty years
How can we understand the above chronology?