Now I wonder how careful the biological grandparents, great-aunts and uncles of the OP’s four step-grandkids’ other step-siblings, with whom they live, are to spend equally on their own two grandchildren, and their four step-grands, and hey, surely they should think of their daughter-in-law’s ex-husband’s two children with his second wife…
If my DD marries her boyfriend, and his son becomes my step-grandson, he will still get as much as he’s been getting from his own bio grandparents, my DGS will still get what he’s been getting from me (which isn’t much, I spend £30, not £300 or £3000).