My sister in law is in a shelter with 2 children and has a drinking problem. Her mother says she will take custody of children but will have a hard time financially to raise. What steps can we take to get financial help for grandmother and can we get mother in a free rehab and counciling facility out of state ?

First no, there are almost no "free rehab" centers but they can check with their state welfare workers as to any assistance available.

The person who is going to have the children will need to get the parents to sign over guardianship to them. If the mother can not raise them, the father would have the next right to do so. If he does not want to, then guardianship can go to another family members.

They would need to go to the welfare office and apply for what assistance is available.
Normally the mother and father of the children would be expected to pay child support

If the mother can't take care of the children, where is the father? You say sister-in-law, which would indicate your brother is the father. If the mother can't care for the children, the state will try to find foster care. They will show preference to the family.

If they are currently in a shelter she probably has a case worker. Have you or your mother tried talking to the case worker?