It’s not halal and they won’t be legitimate I would rather have no children than them to be illegitimate progeny. Since I have many many siblings I would take care of their children and treate them like my own.Would you allow for a surrogate?
Most people want children for them to make dua for them when they are in their grave. People collectively make dua for all their ancestors who died as Muslims whether direct or not such as grandparents grand uncles and grand aunts, they don’t differentiate between them.