I Dont Understand It

What in Tartaros does religion have to do with this?

Thats easy, methamphetamine + religion = Ted Haggard.
Then again I do know people who have found religion good in getting off the downward spiral.


As for this topic GoldenRivet, there is only really one way . It may not be pleasant and it will cause lots of trouble with the family(but that exists anyway by the sounds of it).
Your in-laws will have to get their daughter reported to child welfare as an unfit mother and have the children made wards of court by the State and taken into care.
Then apply as next of kin(unless the childrens father who you do not mention applies) for legal guardianship.
It is a long process which can be complicated and expensive, and may not even get the desired result, but unless they can get a voluntary agreement with their daughter it is the only viable option.