My husband has been living with his paramour and her children for over a year. I have had de-facto custody of my 14 and 16 year olds since 1/13/12.

If by custody, you mean possession, courts like to maintain the status quo in situations like that which means they are likely to keep things the way they are.  However, the overiding consideration is the well being of the children and it's impossible to know for sure what will happen without knowing all the facts.