YuYan was thinking about what she can do for her beloved baby brother to be with his wife and son. She will not let anyone try stopping Xiao XiRan being with his own family.
She hated that there's a law that forbids the existence of a mixed blood. YuYan believed that all creatures are beings with someone they love.
Even though they have differences; as long as they cherish each other, there should be no prohibitions. YuYan has confidence that having mixed blood means unity with other creatures.
Furthermore, YuYan is not different from her baby big brother; they're both mixed blood who wants being with another Hùnxie. Then why is she allowed but Xiao XiRan is not?
Besides, marrying another Hùnxie will only result having a child with mixed blood too, it doesn't make any sense to forbid one but allow the other. That's bias.