This is a bit of an occasion -- the first time, to my knowledge, that SGM has examined the royal ancestry of someone with whom I'm socially acquainted. China and I have had a number of friendly professional and social interactions over the decades; most recently, early in 2023 I published a new edition of his 1998 debut novel KING RAT, with a new introduction by Tim Maughan.
I don't know anything in detail about China's family, but I can add to Olivier's post below that his mother Claudia Cecilia Vaux Mieville died in 2007, as mentioned here:
https://www.theguardian.com/books/2011/may/14/china-mieville-life-writing-genre
I do note with some delight that if Olivier's descent is correct, China is a direct descendant of the remarkable Lady Mary Wortley Montagu (1689-1762), poet, diarist, prolific correspondent, and advocate of smallpox inoculation,
through her daughter Mary who married James Stuart, earl of Bute.