Need advice from Step Fathers/Fathers please
Two things.
Be firm but fair.
And.
You cannot be a good parent and a best friend too. One or the other.
I know a lot of people may disagree but over the years raising kids and step kids (a term I despise) you really have to be a parent first and foremost.
This is not to be confused with you can't hangout and have a blast with them, because you can.