is Kylo Ren's father, then how is
Darth Vader Kylo Ren's grandfather?
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Kylo Ren has two parents: Han Solo and Leia Organa.
Leia is the daughter of Anakin Skywalker/Darth Vader, making Kylo Ren the grandson of Anakin/Vader.
Leia refers to their son a number of times in conversation with Han.
He met her eyes steadily. “We’ve lost our son, forever.”
“Always,” she told him. “From the shadows, in the beginning, even before I realized what was happening, he was manipulating everything, pulling our son toward the dark side.
“If you see our son,” Leia whispered, “bring him home.”
A maternal grandparent is someone who is your mom's parent. Your paternal grandparents are your father's parents. Your mom's parents are your maternal grandparents.(source)
Another way of saying it is "my grandfather on my mother's side" (or maternal grandfather) vs "my grandfather on my father's side" (or paternal grandfather).
You are correct that Vader is not Kylo's paternal grandfather (as that whould be Han's dad, and Vader is not Han's dad), but Vader is still Kylo's maternal grandfather (as that would be Leia's dad, and Vader is Leia's dad1).
Here's a chart I made to help make it abundantly clear:
1 We know Vader is Leia's father from Return of the Jedi, when Luke learns that Leia is his twin sister.
LUKE: Leia's my sister!
KENOBI: Your insight serves you well.