Prayers for the well being of all involved.
Hopefully, if your sister-in-law really does have specific problems, she will accept help. There are definitely times when the mother is not the best care-giver and this should never be assumed.
Special prayers for your brother. This has been tough on him and will continue to influence how he looks at the children's mother. He will need God's special grace to be firm in keeping the chilren safe while still having Christian charity toward the mother. Not an easy job.
nma