FOR THE CHILDREN'S SAKE:
If you are in a better position (financially and or morally), you can do these:
Do your best to shield the children (4 of them) from the discouraging and confusing (to them) situation of their family by spending some time with them to help them forgive the faults of their parents. Responsibly lead them to Christ and to live a Christian life.
Have a sincere talk with your brother-in-law for him not to neglect his children. Talk about how he can provide for them without the money flowing to your sister unless he likes to provide also for his undeserving wife.
Guide your sister into becoming the best mother as she is supposed to be and a remorseful and changed wife (even if the chance that her married life will be back to an ideal kind at present). Always pray for their family to become the way the Lord wanted them to be and plead for them as you love them all - sister, brother-in-law, nieces/nephews. I hope you can relate with these advises.
THE POSSIBLE LEGAL STEPS:(not the best IMO as far as your question on the sake of the children)
Your brother-in-law may be sued in any/combination/all of the following:
1. a case for abandonment and support;
3. Violence against women and their children;
2. Concubinage if he lives with the other woman in a manner qualifying his acts under that offense (based on your narration, there is no concubinage yet.)
Be warned however that a suit against your brother-in-law might not satisfy your desire for the sake of the children. Examples -
1. They might be called to the witness stand (case against the father or the mother) to testify for any of their parents and further scar their life with the memories of quarreling parents and relatives.
2. Even when money might arrive as support, the same will be given grudgingly in contrast to how the same amount might arrive if you were able to gently remind and convince the father to support the children.
MY WORD FOR YOU:
Nandiyan ka na rin lang na concerned para sa mga pamangkin mo, lubos-lubosin mo na ang pagtulong sa kanila. God bless you.
Last edited by rchrd on Mon Aug 01, 2011 10:13 pm; edited 1 time in total (Reason for editing : for clarity)