My sister is married and has 2 adult children from a previous relationship and her husband has 4 adult children from previous relationships. They have property together and live in Louisiana. There is a will in place which her husband has willed his share of the property over to his stepdaughter. In her will she has willed her share of the property over to her two children. Is there anything the children of the husband can do according to Louisiana law to obtain their father's share of the property even though he has a valid will?

Has he died or is he still living?

The husband would have to die before the wife, if he dies first, it goes by his will, then it is her estate if she dies after him.