All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The stepchildren do not inherit from their stepmother if she dies as there is no cause for inheritance between them and this woman. The causes of inheritance are lineage, slave-to-master relationship and marriage. Marriage is a cause for inheritance of the husband from his wife and the wife from her husband.
Allah knows best.