"Mmm." Riku turned and gathered the Prince up into his arms comfortably. "She and I talked today. About our families, about our pasts. And... we decided, since neither of us have families anymore, that we might be family to each other."
Riku leaned down and kissed the Prince's forehead. "When we marry, you too will gain a sister-in-law." Fictive, maybe, but that didn't matter.