New Step Mom

I have a step son who is 5 years old and is just great.  And I just had a baby about 3 months ago.  I have been having anxiety because my husband and I will agree on household rules... Then I will stick with them and he will let things go and not follow these rules.  This makes me the bad guy and him the fun dad.  Am I over stepping my bounds as step mom by being upset by this when he isn't "my kid"?  I want us all to be a family but it seems like it's just the two of them setting the house rules when he has his son here.  It also worries me because I want this new baby to have certain rules and not have a different set of expectations for each parent. I am pretty new to the whole step mom role so any advice would be appreciated.