Chatoyer

Visenya Targaryen was promised at birth to her cousin, Maelor. But, in a few twists of fate, she is betrothed to another, a man she has never met and to whom she certainly does not wish to bind herself. Her betrothed, acting only on his father's wishes, comes to Dragonstone to claim his bride.

This is an age gap, betrothal, Targcest, uncle/niece pairing that will be inherently toxic due to the characters and nature of their relationship. If you don't want to read about that, now is the time to see yourself out.