@SleeplessInWherever I don’t think it’s as black and white as in the family or not - especially with step families.
OP’s daughter doesn’t have Parental Rights. She hasn’t legally adopted these children. They have a mother and father. They are the parents.
thier mother and father have a legal and moral obligation to support them financially. They have a step mother and a step father who have no such obligation. They have a moral obligation to be kind and welcoming. But their income is not taken into account for child maintenance because they are not the parents.
clearly their step father feels no such obligation given he isn’t working so must not be able to support his own biological children never mind his step children.
SIL allowed a situation where his wife funded private education for two of his six children and therefore i assume was content with any issues of fairness. If he wants his children to now have a private education then that is for him to fund. If their mother also wants this she needs to find employment and contribute. It isn’t for OP, her daughter, the children’s step father or indeed his parents to find this money.