Does my brother in law have rights to receive arrears in child support?

That money belong to the father in law. The children never was intitled to anything other then thier care from that money. As a child you don't take the money and hand it to your child you pay for his clothing, food, housing, and any other needs your kids may have. He has already done all of that with his own money...

Now after all of these years he finally got paid back that money should be taking care of him. I am sure he did without alot for his kids as they grew. Now he should be taken care of it is his time...

You said it all when you said "arrears". Arrearage is just that, money that was owed a long time ago that was never paid. Money that dad should have gotten when the children were under 18, to take care of them while they were young. Since dad did not get it then, then he was forced to use extra of his own money to provide for what mom should have provided for back then. It is sorta like being reimbursed. Dad has no "legal" obligation to give any of that money to the children now that they are grown. If he had gotten that money back when he was supposed to, he wouldn't have handed his kids a check then would he have?? Morally he can do whatever he chooses. But legally he does not have to give his kids that money now.

The father raised the child. The support was to help keep a roof over the childs head and meet the needs of the child. The father is entitled to all monies received. He is not bound by law to share it with anyone.