After being married for 18 years and purchasing our home together will i lose it to his grown children if he passes away

The *likely* answer is that you will own 100% of that house when he dies. Most deeds for married purchasers are put in the name of "tenants by the...

It would depend on state law, which would define "issue". In Maryland, the answer is no - step children do not count as issue until adopted.

Just to add an extra 2 cents to the other comments - if the property is held in your father's name, instead of a trust, his interest will pass...