If you need treatment because of your serious illness, and your brother's family can afford the relevant expenses, if your sister-in-law refuses treatment, you can sue.
If it is difficult to bear the cost of treatment because of the need to raise children and the difficulties in family life, there is no hope of prosecution, and the law cannot force your sister-in-law to do her moral duty. If you use morality to hold your sister-in-law accountable, you have no responsibility as a younger brother.