The other 3 siblings must be older than Griffin.
I was thinking how Griffin must have foud out about their existence. Presumably, by somehow getting access to the legal records of guardianship, or other records of sorts.
But Martlet, Oriel and Rook could only leave traces behind if there was time and place to leave traces behind... Had they been younger than Griffin and Robin, they would still be in China and therefore have no records. So how could Griffin know about children who were still in China? Seems highly unlikely, unless he managed to access letters/transactions made to the maids that Lovell always kept with the children in China (which... doesn't make a lot of sense according to the timeline and Robin didn't find correspondence of sorts when he went into Lovell's stuff).
All of those people have to be older than Griffin... If not older, then raised away from Lovell (London/Oxford) but so what for?
This also leaves the question of, how freaking old is Lovell???? Because for all of those to be older than Griffin, he has to be OLD. Like, much older than what I envisioned.
I also have a headcanon that there must be another kid in China who will finally not meet the same fate. I believe that in the same trip in which Lovell got Robin, he took the change to impregnate another woman. This would mean that this child would have been SO CLOSE to have been taken to England but Robin broke the cycle... The child will keep his name, his identity, his language. This child will grow and live in his motherland because they broke the cycle...