It makes perfect sense with his character. Kylo is a brat, a brat born to a "royal" family. He's the first born of the next gen and meant to be special like his grandfather. He's heard good and bad about his grandfather and are intrigued by both. He starts asking unnecessary questions about the darkside and this scares his family. Starts training with his legendary uncle and is the apple of his eye...until his baby cousin is born and conveys way more potential than he does and his uncle is enamored by his new born child. Grandson gets extremely jealous, gets told about how great his grandpa was using the darkside of the force by some evil old family acquaintance, starts committing to the dark side and committing bad acts. Wants to kill his baby cousin who getting all this attention but leaves her alone because he doesn't see the hooplah. Uncle gets sad that he allowed his nephew to go bad under his eye. Doesn't want the same to happen to his daughter, leaves her on a desert planet with watchful protectors around while going out to search for answers since he knows the ultimate battle of light and dark will be within his family and that they are essentially gifted and cursed. He knows she'll search for him when times are darkest because he knows his daughter is more than just "special", she's chosen....
You're welcome.
.
