Your challenge is to have an Iranian people serve as the royal and noble family of a state in modern China, excluding Tibet and Xianjiang. They don't have to actually control China - they might simply be a claimant to the Mandate of Heaven during one of the country's many 'broken up' periods - and they don't have to speak Persian, although that would be cool. They simply be have to be an Iranian people, who at least start off speaking an Iranian language, that rule an area of China.
I think the easiest way would be some kind of eastward Scythian migration who end up settling in the region, but if you can think of a more interesting way I'm all ears.
I think the easiest way would be some kind of eastward Scythian migration who end up settling in the region, but if you can think of a more interesting way I'm all ears.