Maintenance allowance for children

Sir, My younger sister died of cancer three years back.Her husband is working in health department of Kerala government. They are having two children. Elder daughter now 17 years, just completed plus two and now thinking of going for higher studies. Younger son aged 14 is 9th standard student. Husband was a drunkard and sister had to suffer a lot before her death. The ailment itself could not be detected due to her family problems because of poverty and harassment etc. She had even complained to the police regarding this a few years back but apart from some advice they could not do anything. After the death of his wife the children were under the care of their uncle. Their father gave some paltry amount whenever he wished. And for the past 6 months that also stopped. Recently he is living with another lady, a mother of two. Now we do not have any hope of getting help in the future for the lively hood of the children and their education. Can we proceed for maintenance? If so what is the procedure?