Jade, I'm sorry that your experience with your stepmom turned you sour to step parents.
As a stepmom, I feel compelled to reply to you. For the most part, I try to stay out of things, and I let DH do the talking to BM. However, my opinion does matter, I just don't usually talk directly to BM.
Here's the thing, when it comes to decisions for parenting SS, my opinion does get considered, b/c DH and I also have 2 children together. We try to be consistent with all the kids, but also be consistent with BM's house as well.
Finances also get discussed between us. Yes, CS is non-negotiable, just like the mortgage, or electric bill. However extras (like when DH was not paying bills so that he could foot the daycare bill that BM was responsible) are up for discussion.
As far as taking care of kids, sometimes the parents leave it up to the step parents to do some of the parental responsibilities. When BM was working, it fell to me as the step mom to take care of SS when ever he couldn't go to daycare (usually b/c of illness). I picked out the daycare that he attends. When BM was responsible for SS's health, she fell behind on his shots and his dental. Now that he lives with us, I make sure that he is up to date on all things medical.
I also drive SS to school EVERYDAY, take him to his soccer practice 90% of the time (DH takes him the other 10%), and attend all his games. I chaperone his field trips, I attend his school functions. DH and BM attend some of the functions, but not as many as I do. (Keep in mind I also work full time while BM is a stay at home NCP. She has the ability to attend all of SS's functions, but chooses not to)
I may not be SS's mom, but I am his parent. BM may not like it, and she may be downright nasty about it, but that is her problem not mine. We used to be able to speak amicably, but when she decided that she couldn't handle that, I let DH deal with her. Yes, there are times that it would be easier for her to speak directly to me (especially when it comes to scheduling anything), however b/c she is so difficult, DH and I have decided that I won't speak with her unless absolutely necessary. And now, even if she has a problem with me, she discusses it with DH,not me.
I am not trying to replace BM, and I made that clear from the start. In the beginning, I let her dictate how involved I was, and I've followed her lead which is to make me as responsible if not more so for her child. This was her choice, but I am a willing participant.
Note to Bleemom- it may not be a bad idea to remove yourself from the situation for a bit. Yes, BM lied, yes you caught her. She knows that and so do you. Things are probably going to be tense for a bit, so now would probably be a good time for DH to step in and do the communication.