You also have to talk to your brother-in-law. These are his children and they are not your responsibility they are his. He should be reassessing his plans knowing he will become a single parent. I would not do everything for the children and try to get him to do some stuff. Perhaps there is a social worker at the children’s school and you could meet the 3 of you to discuss supporting the children through a difficult transition time. There may be a social worker at the cancer clinic if the school isn’t an option. Although you are doing all the caregiving, you aren’t their guardians which could create issues if you had to go to the doctor, sign school forms, etc.