Try to have third kid

I have a 7 year old and a six year old from my other relationship, I'm trying to have new baby, in my relationship that would make us have 4 kids together since she haves , a son almost 2 years old, sometimes things get stressful life, money her baby's father doesn't help much, I love them both, we with have ups and downs, I'm really looking forward our life together. I guess my question is how do you balance stepkids with your OWN kids with a new kid? and with her son that lives with us how do i parent when I'm not really the parent a lot of times I feel it's not my place to discipline or anything like that