Hello,
1) Unfortunately, you can't do anything legally to stop the marriage as law permits to adults not within prohibited degree of relationship to marry. And besides they are already married in a temple.
2) What can be done is, your mother can file a case in the Magistrate court seeking maintenance from your brother especially as he got the job on compassionate grounds after your father's death.
3) If the sisters are unmarried they too can file for maintenance against your brother.